BACKGROUND ART Anti-fouling coatings that prevent and retard the attachment and growth of organisms on surfaces immersed in marine environments are numerous and old in the art. In general, their effective properties are derived from the incorporation of biocidal agents such as cuprous, zinc and titanium compounds which are slowly solubilized and released into the marine environment. The relative effectiveness of such coatings depends on the amount of biocidal agents incorporated into the coating. The larger the amount of a biocidal agent, the greater is the rate of release and the more effective is the anti-fouling mechanism. As concerns regarding the release of biocidal agents into the environment have heightened, however, other methods for achieving the desired anti-fouling properties in marine coatings have been investigated.

Anti-fouling coatings that depend on the slow release of biocidal agents have another disadvantage. The duration of their anti-fouling mechanism is limited by the amount of biocidal agent that is incorporated into the coating for continuous release. Once all of the agent has been

released into the marine environment, the coating is no longer effective, and a new coating must be applied. As observed by others working in this art, the maximum effective life of an anti-fouling coating that releases a biocidal agent is two to three years. Because of this inherent limitation in the effective life of anti-fouling coatings that release biocidal agents, other methods for achieving the desired anti-fouling properties in marine coatings have been investigated.

DISCLOSURE OF THE INVENTION Coatings of this invention solve the foregoing problems posed by biocidal agents by employing an entirely different anti-fouling mechanism - the electric potential that is created when a metal such as gold, silver, or copper behaves galvanically in an aqueous environment. Mild electric fields are effective in repelling organisms that foul the surfaces of immersed structures. This result is illustrated in United States Patent No. 5,344,531 to Saito, et al., which discloses the disruptive effect of a continuous or intermittent direct electric current on the action potential of marine organisms at the nerve-muscle interface.

Unlike the teachings of Saito, et al., however, coatings of this invention do not require an external power supply to generate the desired electric field.

Rather, a metal such as gold, silver, or copper will act as a cathode in relation to the dissolved oxygen, hydrogen and other compounds in the aqueous environment, which act as electron donors. Water - distilled, fresh or marine - serves as the electrolyte. The resulting electric field is disruptive enough to prevent the attachment of fouling organisms. Steel plates which

the inventor experimentally treated with a copper coating of this invention and submerged in marine waters off the Florida Keys remained free of fouling organisms after two years of exposure.

The inventor has hypothesized that the hulls of seafaring vessels from centuries ago that were clad with metal plates or foils resisted fouling by organisms because of this mechanism. Metal plates and foils, however, have become impracticable and incompatible for use with the hulls of modern vessels. Relatively recent advances in the milling and manufacture of metal microspheres finer than 300 mesh enable the effective incorporation of a metal such as copper into coatings that are suitable for use with the hulls of modern vessels. The metal microspheres are dispersed and set within a matrix consisting essentially of a polymeric binder such that the resulting coating exhibits the desired properties of a sheet or foil of the same metal. Although metal powder coatings, as a class of materials, are not unknown in the art, the incorporation of metal microspheres into coatings of this invention is new.

Coatings made in accordance with this invention may be designed for one or more specific applications. One would have to take into account the surface being treated and the environmental conditions to which it is exposed, e.g., degree of turbulence, levels of ultraviolet radiation, and the duration of submergence, in order to select a coating that is the most stable and durable on that surface and under those conditions. The preferred embodiments of this invention are specifically designed for use with the hull surfaces of vessels. Its intended use dictates the following additional objectives:

1. To provide an anti-fouling mechanism that is relatively long- lasting, thereby eliminating the number and cost of periodic cleanups needed to remove fouling organisms from the hull surface and minimizing the number and cost of reapplying coatings needed to maintain the effectiveness of the anti-fouling mechanism.

2. To provide an anti-fouling mechanism that will not adversely affect the hydrodynamic properties of the hull surface.

3. To provide an anti-fouling coating that is easily applied and removed, thereby permitting widespread and inexpensive use on vessels having a variety of hull surfaces.

4. To provide an anti-fouling coating that also protects the underlying surface, if metal, from the corrosive effects of electrochemical reactions occurring within the aqueous environment.

These and other advantages of the invention will be apparent from the best mode disclosure and the industrial applicability sections that follow.

BEST MODE FOR CARRYING OUT THE INVENTION As described above, coatings of this invention are compositions having a polymeric binder chosen for specific physical and chemical properties and characteristics, in which is dispersed an amount of metal microspheres.

Submerged in an aqueous environment, the metal microspheres behave as a cathode relative to the dissolved oxygen, hydrogen and other compounds, which act as electron donors. The water, distilled, fresh or marine, acts as the electrolyte. As a result of the electrochemical reactions between the copper and dissolved oxygen and hydrogen within the aqueous environment, a mild electric field is created that repels organisms which would otherwise foul the coated surface. Such coatings also protect the underlying surface, if metal, from the corrosive effects of the electrochemical reactions occurring within the aqueous environment.

The effectiveness of such coatings depends on two critical factors - achieving maximum, even dispersion of the metal microspheres within the liquid matrix comprising a polymeric binder; and selecting maximum stability and durability and optimum adhesive properties in the coating, based on the surface and structure to be treated and the environmental conditions under which it is to be exposed. The invention will work as disclosed and claimed herein so long as the metal microspheres are adequately dispersed within the polymeric binder, and the coating is stable and durable enough for the intended application and adheres to the surface being coated. Thus, a particular coating may vary, without departing from the spirit of the invention, in the polymeric binder selected, the matrix selected, the mesh of the metal microspheres, and in other properties, if such variations are necessary to make a coating that is suitable for a particular surface of a particular structure within a particular environment. For

example, a coating to be applied to a stationary object such as a buoy can tolerate a coarser mesh of microspheres than that suitable for a coating on a vessel hull because the hydrodynamic properties of the buoy are not critical.

Noble metals, namely, gold, silver and copper, are most suitable for coatings of this invention because they do not easily react with molecular oxygen to form oxides. Other metals, namely, titanium, iron and zinc, could be used but they readily form oxides which cause a layer of slough to form over the coating, thereby inhibiting the chemical reactions needed for the anti- fouling mechanism. Because the use of gold or silver is prohibitively expensive and neither metal is readily available, copper is the metal of choice for coatings of this invention and the metal used in the preferred embodiments disclosed below.

Although copper metal microspheres of a substantially uniform size falling within a range from 50 to 325 mesh could work in coatings of this invention, the ideal size range for the microspheres is 175 to 325 mesh. Not only will microspheres in the lower mesh range, below 175 mesh, create dimples in the coating after the same has been applied to the surface, but their dispersion within the polymeric binder will be more uneven. The resulting coating will require continuous stirring and mixing during application to maintain maximum dispersion of the microspheres. Microspheres approaching the fineness of talcum powder (@400 mesh) are equally difficult to work with because the particles tend to trail during mixing with the matrix.

A homogeneous mixture is therefore difficult to obtain.

The preferred embodiments described below relate specifically to a coating suitable for the hulls of vessels. Copper metal microspheres purchased from United States Bronze Powders, Inc. were used. Around 15% of the microspheres fell within the 200 to 325 mesh size range and around 82% of the microspheres were at 325 mesh in size. The copper metal microspheres were combined with a liquid matrix comprising a polymeric binder in a 1:3 ratio by weight. Thus, three pounds of copper metal microspheres would be combined with nine pounds of liquid matrix to produce a total of twelve pounds of coating. This amount is equivalent to about one gallon in volume of coating. Ratios ranging from 1:9 up to 5:9 will also work, but 1:3 is preferable.

Numerous polymeric binders, including acrylic latex, polyurethane, and epoxy resin, were experimented with to develop and select a coating having the following preferred characteristics: 1. The coating would be stable in distilled, fresh or sea water.

2. The coating would not break down from prolonged exposure to visible spectrum light and to ultraviolet radiation.

3. The coating could be applied easily in room temperature, i.e., temperatures 40 degrees Fahrenheit or higher.

4. The coating would be effective for a reasonably long period of time.

5. The coating would be stable in conditions of either intermittent or continuous immersion.

6. Neither the coatings performance nor its anti-fouling mechanism would be affected by the surface being coated.

Rather, the coating would be suitable for use with wood, metal and fiberglass hulls.

One matrix consisted essentially of ENTERPRISE acrylic latex in a satin white, interior/exterior enamel coating #3330 48967 containing crystalline silica. A test sample was made by combining 50 grams of copper metal microspheres with 150 grams of the liquid matrix, with the metal being added in ten increments of 5 grams each interspersed by 30 seconds of stirring. The coating was applied to a preformed steel plate in four coats. (As a general rule applicable to all coatings, surfaces to be coated should be cleaned, dried and kept free of dust, dirt, moisture, grease and loosely adhering particles.

Rust should be removed. The surface should be sanded or roughed to maximize the area of contact and adhesion with the coating.) The first coat (base coat) was a 10-mil thick layer of the liquid matrix without the metal microspheres. The second coat was a 20-mil thick layer of the prepared coating containing the metal microspheres and the third coat was a 40-mil thick layer of the same. The balance of the test sample was applied in the fourth coat, and the coating was then allowed to cure. The coat cured after six hours at room temperature.

In use, the acrylic latex matrix was found not to be durable enough.

The matrix softened in water after 24 hours. The coated sample required mild abrasion of the surface with fine sandpaper or steel wool to expose the copper

metal microspheres to the aqueous environment. The coated sample exhibited, however, a measured electric potential of 640-650 millivolts when submerged in distilled water. A minimum potential of 50 millivolts is required to achieve the desired anti-fouling effect.

Another matrix consisted essentially of EVERCOAT fiberglass epoxy resin in a colorless finish coating #499. A test sample was made by combining 50 grams of copper metal microspheres with 150 grams of the liquid matrix, with the metal being added in five increments of 10 grams each interspersed by 30 seconds of stirring. Then 60 drops of methyl peroxide (at a ratio of 12 drops per ounce of coating) were added to the coating. The coating was applied to a preformed steel plate in five coats. The first coat (base coat) was a 10-mil thick layer of the liquid matrix without the metal microspheres. The second coat was a 20-mil thick layer of the prepared coating containing the metal microspheres, the third coat was a 40-mil thick layer of the same, and the fourth coat was a 100-mil thick layer of the same. The balance of the test sample was applied in the fifth coat, and the coating was then allowed to cure. The coat cured after 30 minutes at room temperature.

In use, epoxy resin was found to harden too quickly, thereby making it difficult to work with under room temperature conditions. The use of methyl peroxide created an exothermic reaction that made the coating more difficult to handle. The resulting coating would tend to blister and crack. The coated sample also required mild abrasion of the surface with fine sandpaper or steel wool to expose the copper metal microspheres to the aqueous environment.

The coated sample exhibited, however, a measured electric potential of 640- 650 millivolts when submerged in distilled water.

Polyurethane was the most workable polymeric binder. In an earlier embodiment, the matrix consisted essentially of an ENTERPRISE oil base with a high polyurethane concentration in a #686 regatta red, oil gloss enamel paint. This matrix alone did not provide the desired properties of durability and adhesiveness, however. To solve this problem, in the preferred embodiment, the ENTERPRISE oil gloss paint was combined with PL 185TM Wallboard Adhesive, a waterproof rubber-base adhesive, in a ratio ranging from 1:1 to 3:1 by volume. Because the presence of the adhesive significantly improves the loading characteristics of the matrix, a ratio of 1:1 by volume is preferred.

To make a test sample, 50 grams of copper metal microspheres were added to 150 grams of the matrix in one step with three minutes of continuous stirring. The 20-mil thick layer of the coating was applied to a preformed steel plate. The coating required only normal stirring prior to use, while the other compositions required continuous stirring during application.

One layer of the coating was sufficient, and no base coat was needed.

Although the coated surface was usable after 24 hours, a drying time of 48 hours and a curing time of seven days within a temperature range of 40 to 80 degrees Fahrenheit are preferred.

In use, the coated plate exhibited a measured electric potential of 300 to 700 millivolts when submerged in distilled water. Abrasion is not necessary

with this embodiment.

INDUSTRIAL APPLICABILITY As already noted above, numerous industrial applications for an anti- fouling coating of this invention can be conceived by one skilled in this art.

Most commonly, coatings of this invention would be applied on the hulls of vessels and the surfaces of submerged structures, such as buoys and piers, to prevent and retard the attachment and growth of fouling organisms such as barnacles and mussels. One skilled in the art could devise suitable formulations for a coating of this invention for application on submerged pipes and vents that need to be kept free of blockage by fouling organisms such as zebra mussels. Coatings of this invention avoid the environmental hazards and limited efficacy of anti-fouling coatings that depend on the continuous release of biocidal agents.