Coating processes – Spraying
Reexamination Certificate
2002-02-04
2004-03-02
Pianalto, Bernard (Department: 1762)
Coating processes
Spraying
C427S383500, C427S407100
Reexamination Certificate
active
06699528
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a process for preparing corrosion-resistant articles for marine use, and the products prepared by the process.
2. Background Art
Metal articles and structures are widely used in the marine field, particularly in pleasure craft, fishing boats, and the like. Unless stainless steel or certain marine bronzes are used to manufacture such articles, corrosion, with its associated aesthetic problems and failure modes can be expected to severely limit product lifetime. Even when these two relatively corrosion-resistant classes of materials are used, corrosion may still take place, particularly in salt water or brackish environments. It is partially for this reason that pleasure boats operated in salt water have considerably lower resale value than similar vessels operated in fresh water.
Corrosion problems are most severe when more active metals such as magnesium, aluminum, and carbon steel are used. Such items may become severely corroded over relatively short periods of time in salt water environments.
To lessen the corrosive effects on metals, it has been common to provide surface treatments. Chrome and nickel plating have been used, for example. However, plating is relatively expensive, particularly when large fabricated structures constructed by welding are to be plated. In addition, such plating procedures do not work well on many active metals such as aluminum.
Anodizing has also been used to increase corrosion resistance, and is effectively used on small parts. However, large tubular structures such as radar arches are typically welded together. The anodized coating is destroyed locally during the welding process. Anodizing very large, prefabricated structures is not cost-effective.
Painting has long been used to provide corrosion resistance, and literally hundreds, if not thousands, of coating systems have been proposed. Powder coating systems, for example using epoxy-type powdered resins which are subsequently heat cured can produce excellent finishes. However, powder coating of large articles becomes expensive due to the size of the cure oven necessary. Moreover, powder coating systems exhibit the same corrosion deficiencies associated with other coatings.
Use of conventional, solvent-borne coatings is becoming increasingly difficult due to environmental legislation limiting emissions of volatile organic compounds (VOCs). Moreover, most such coatings do not provide the necessary levels of corrosion protection, and application to substrates involves a time and labor-extensive combination of primer application, smoothing filler, and topcoat application, with numerous sanding and smoothing steps in-between.
A problem with coatings subject to corrosion occurs when a prefabricated structure is altered to mount to non-standard surfaces or to mount additional components thereon. For example, flybridges and radar arches are often used to mount flag holders, GPS and RF antennae, hand holds, cleats, “rocket launcher” fishing pole receptacles, and the like. Drilling the necessary mounting holes into the coated structure penetrates the coating, exposing untreated metal to the environment. Corrosion rapidly occurs at such areas, frequently spreading between the coating and its metal substrate causing ultimate separation of the coating.
It would be desirable to provide a process by which metal (and non-metal) articles slated for marine use could be coated with a corrosion-resistant coating employing a minimum of finishing steps. It would further be desirable to provide a coating which offers extended corrosion resistance even after having had bare metal exposed.
SUMMARY OF THE INVENTION
It has now been surprisingly discovered, that a two-part polyurethaneurea coating derived from a low viscosity isocyanate-terminated prepolymer and a mixture of diamine and hydroxyl-functional curing agents can be applied to marine products by conventional spraying techniques to produce a smooth, corrosion-resistant coating which maintains its corrosion resistance even after exposure of bare metal to the environment by penetration of the coating, and even under salt spray conditions.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The process of the present invention involves coating marine articles with a two-part coating polyurethaneurea composition at elevated temperature, and allowing the coating to cure to a corrosion-resistant film.
The articles to be coated include both metal and non-metal articles to be used in marine environments. Preferably, the articles are metal items used as fittings and structures on watercraft. Non-limiting examples include cleats, plates antennas, antenna mounts, radar arches, bow rails, rub rails, fishing pole holders, downrigger mounts, rails, stanchions, swim platform supports, exhaust ports, mast bases, back stays, chain plates, transom, swim, and boarding ladders, tuna towers, fishing platforms, flybridges, anchors, anchor rollers, and the like. The metal substrate may be any metal, for example brass, bronze, bright metal, zinc, magnesium, aluminum, steel, stainless steel, and the like. Preferred are steel (non-stainless), stainless steel, chrome-plated metals, aluminum, and magnesium. Terms such as aluminum, magnesium, bronze, etc. include the various alloys of these metals. Preferred non-metal substrates are thermoplastic sheet material and fiber-reinforced thermoset and fiber-reinforced thermoplastics. Fiber reinforcement of the latter includes fiberglass, thermoplastic fiber, carbon fiber, ceramic fiber, etc., whether in the form of strands, tow, yarn, woven or non-woven products, felted products, and whether the fiber reinforcement is short fiber, long fiber, continuous fiber, etc.
The two-part polyurethaneurea coating is supplied in at least two components to a spray gun. Although it is possible to supply three or more components, it is simple and most economical to supply the components in what are conventionally termed “A-side” and “B-side” components, the A-side containing the isocyanate-functional components and the B-side containing the isocyanate-reactive components.
Both the A-side and B-side desirably have viscosities which are below 500 cp at the spray temperature in order for sufficient mixing and atomization to take place. However, higher viscosities are possible provided suitable mixing and atomization can be achieved.
The components are preferably VOC-free. By VOC-free is meant that no added solvent is employed. The coating components are substantially 100% solids. However, it is possible to add minor amounts of solvent, particularly when component viscosity is higher than desired. In such cases, addition of up to 20 weight percent solvent, more preferably 10 weight percent or less, and most preferably 5 weight percent or less of solvent may be practiced. When solvent is added, it is preferably that the solvent be a “zero-VOC” solvent, i.e., a solvent which is not viewed as contributing to environmental problems. Such solvents are known, for example, in U.S. Pat. No. 6,048,471. An example of such a solvent is &tgr;-butylacetate. Mixtures of such solvents may also be used.
The A-side of the polyurethaneurea coating is an isocyanate-terminated prepolymer prepared by reacting an excess of a diisocyanate with a difunctional polyoxyalkylene polyol. The isocyanate is in excess such that an NCO-group content of preferably from about 2 weight percent to about 12 weight percent, more preferably 4 weight percent to 8 weight percent is obtained. The diisocyanates employed are preferably mononuclear cyclic diisocyanates such as isophorone diisocyanate or the toluene diisocyanates, preferably a commercial mixture of 2,4- and 2,6-toluene diisocyanates. While straight chain aliphatic isocyanates such as 1,6-hexane diisocyanate may also be used, their use is less preferred. Also less preferred are the polynuclear diisocyanates such as 2,4′-, 2,2′-, and 4,4′-methylene diphenylene diisocyanates. The preferred diisocyanates are an 80:20 blend of 2,4- and 2,6-
Brooks & Kushman P.C.
Pianalto Bernard
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