SPECIFICATION 
     This application is a continuation-in-part basee on prior filed application Ser. No. 543,499 filed Sept. 21, 1983, now U.S. Pat. No. 4,552,813 which in turn was a continuation of application Ser. No. 348,504 filed Feb. 12, 1982 and which is now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The hulls of ships, the surfaces of pilings, and other objects subjected to the activity of seawater inevitably become encrusted with various forms of marine life. 
     The marine life which cause the greatest problem are the crustaceous formations, such as barnacles, attaching themselves to the hulls of ships to cause a significant reduction in the speed with which the ship travels through the water. Among the many developments in this field are antifouling paints that inhibit the growth of such crustaceans, e.g. by incorporating tin and copper compounds in the paint to produce a toxic effect on the crustaceans. These materials however are pollutants that might cause undesirable effects on fish and other seafood which are eaten by humans as well as inducing health problems in persons dealing with the tin and copper compounds. Furthermore, many of the available antifouling paints are effective for only a short period of time necessitating scraping and refinishing at frequent intervals in order to provide the desired protection. 
     In accordance with the present invention a new composition has been discovered which provides antifouling protection more effectively and for longer periods of time than has been known in the past. Since the expense of removing a ship from the water, scraping its hull, and repainting at frequent intervals is significant, it is an important improvement to provide a coating material which will minimize the downtime of the ship and that expenditure of money. 
     It is an object of this invention to provide an improved marine coating composition. It is another object of this invention to provide a method for preventing the growth of marine life on surfaces below sea water by applying to that surface a coating having an effective amount of a novel antifouling agent. 
     It is another object of this invention to provide an antifouling agent which can be added to paint to produce an improved antifouling coating composition. Still other objects will be apparent from the more detailed description of this invention which follows. 
     BRIEF SUMMARY OF THE INVENTION 
     This invention provides a method for inhibiting the growth of marine life on surfaces in contact with seawater comprising covering that surface with a coating in which is embedded an effective amount of particles of an active agent, such as a polycyclic naphthacenecarboxamide. Preferably, the coating is a paint and the embedded particles are tetracycline. Other agents are disclosed herein. Also, preferred concentrations of the agent are disclosed. This invention also provides an antifouling coating composition in which the coating material has dispersed therein an effective amount of particles of an antifouling agent as described above. In specific embodiments of this invention, the amount of antifouling agent is at least about 250 mg. of particles per quart of coating material and the particles are less than about 0.005 inch in diameter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed to be characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which: 
     FIG. 1 is a schematic cross-sectional view of the coating in accord with this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred antifouling agent usable in connection with the present invention is a polycyclic naphthacenecarboxamide in a particle size that is small enough to be readily dispersible in a paint or other coating material. The family of polycyclic naphthacenecarboxamides includes tetracycline, chlorotetracycline, oxytetracycline, fluorotetracycline, bromotetracycline, demethylchlorotetracycline, demethyltetracycline, N-methyl-ethyl-oxytetracycline, 7-bromo-6-demethyltetracycline, 5-hydroxy-d-chlorotetracycline, N-methyl-ethyl-tetracycline, 6-a-deoxy-5-hydroxytetracycline, and their salts, hydrates, phosphates and the like. The preferred compound of this family of agents is tetracycline or any of its closely related derivatives such as salts, hydrates, phosphates, and the like. 
     The antifouling agent is used in the control of marine growth by being dispersed in a coating material which is then applied to a surface that is to be protected from marine growth. The most common application for this use is in an antifouling paint used on the hulls of boats to inhibit the growth of barnacles and other crustaceous formations. Other structures such as pilings, docks, and the like, that are subjected to the action of seawater are usefully treated with the composition of this invention. 
     The base coating materials to which the antifouling agent of this invention may be incorporated is preferably a high quality paint used on the hulls of ships or on any other structure that is subjected to the action of seawater. The coating, however, may be any other type of material such as a varnish, a plastic coating material, a molded plastic structure, etc. It is only necessary that the agent be intimately dispersed in the coating materials so that the agent is available at the outer surface of the coating to inhibit the attachment of marine growth to the surface. In other words to make the agent readily dispersible it is preferred that the agent be ground into a powder of small particle size, e.g. less than about 0.005 inch in diameter, and preferably about 0.001 to 0.004 inch in diameter. When the agent is ground to this small size it is readily dispersible in the paint or coating material and will be available at the surface to prevent the growth of marine life; and will continue to do so as the coating becomes worn or is abraded or flaked away to expose new particles of the antifouling agent that were previously completely embedded and can then provide a continuing activity in preventing marine growth for a much longer period of time than other prior art antifouling paints. 
     It has been found that a very small amount of the antifouling agent is capable of performing the activity of preventing marine growth. As little as 250 mg. per quart of coating material maybe effective to prevent the growth of marine life. In order to provide an even better inhibitory effect it is preferred that the concentration of antifouling agent be between 500 mg. and 1.0 g. per quart of coating material although higher concentrations can be prepared and used. A desirable concentration is somewhat more than 500 mg. per quart. Higher concentrations, such as 1.0 g. per quart of coating material are effective in preventing marine growth and are preferred. Still higher concentrations are at times not preferred because of the additional expense and the increased solids content of the paint. However, maximum quantities in the range of 4 to 8 grams are occasionally desirable. The precise maximum is dependent upon the characteristics of the paint. The vehicle or binder in a paint has a certain upper limit for solids content that it can handle before it begins to deteriorate. Adhesion, strength, and film thickness are all affected by excess solids. In many cases, this is reached at the concentration level of 4 to 8 gram quantity per quart but in many instances the resulting mixture will be a paste rather than a paint. 
     The coating composition of this invention may be prepared by any suitable methods and means which causes the dispersing of small particles of the antifouling agent throughout the basic coating material. In the preparation of an improved antifouling paint composition of this invention it is only necessary to obtain as a basic paint a good marine bottom paint, normally used to cover the hull of the ship, and to incorporate into the paint an effective amount of the antifouling agent of this invention. Any efficient type of stirring or shaking mechanism may be employed to disperse the antifouling agent throughout the paint composition. The paint is then ready to be applied in any known manner, such as brushing, rolling spraying, or dipping, to the hull of the ship or to whatever structure that is to be covered thereby. 
     It should be noted that the basic ingredients of an antifouling paint include the antifouling agent, a vehicle, solvent, and pigment. Accordingly, the present invention is not to be limited to oil based paint or any other type of paint. The vehicle or binder is the film that is left after the solvent evaporates from the surface. The pigment and the antifouling agent are the additives that are caught within the matrix of the coating. In some antifouling agents the binder or film has been chemically linked with the antifouling agent to provide an even distribution throughout the entire surface. These are normally ablative type paints that sloughoff with use. However, the antifouling material of the invention can be suspended in any kind of compatible material to form a coating material that can be applied to a surface or other object such as a net. 
     An illustrative depiction of the painted structure is shown in FIG. 1 wherein the structure itself, e. g., the surface of the hull of a ship is shown at 11 with whatever normal coating 12 that may be found on surface 11. If surface 11 is aluminum, layer 12 might be aluminum oxide. If surface 11 is wood, layer 12 might be a wood finish or a residue of impurities remaining after scraping the hull following a previous period of use in seawater. If surface 11 is fiberglass, layer 12 may be the remnants of a previous coating of paint. Coating 13 is the composition of this invention. It is coating 13 that protects surface 11 from the action of seawater 14. Coating 13 coatings dispersed throughout its volume small particles 15 of the antifouling agent. Many of particles 15 are near the interface of coating 13 and seawater 14 and are continually leached by the seawater to provide an inhibitory action against the marine life in seawater 14 attaching itself to coating 13. It is believed that the long life and continuing effect of the antifouling composition of this invention is localized at the interface between coating 13 and seawater 14 as coating 13 is worn, eroded, or abraded, exposing new particles 15 to seawater 14. This leaching thereby provides the inhibitory effect at that interface. 
     In a series of actual tests to demonstrate the effectiveness of the antifouling agent and composition of this invention, a fiberglass cylinder approximately six feet long and one foot in diameter was sanded and finished with different compositions in four adjoining sections on the outside of the cylinder which was then suspended in seawater. One section of the cylinder was left uncoated, the second section of the cylinder was covered with a quality bottom paint for ships containing 500 mg. tetracycline per quart of paint. A third section of the cylinder was painted with the same type of bottom paint containing the same concentration of sulfanilamide in place of tetracycline. A fourth section of the cylinder was painted with the same bottom paint except that penicillin was used in the same concentration rather than tetracycline. The cylinder was then suspended in seawater in an area near St. Angustine, Fla., where the marine growth problems are particularly acute, and observations were made during the succeeding year and a half. All during that time the second section which contained tetracycline in accordance with this invention remained absolutely clear and free of any marine growth. The two sections that were covered with paint containing sulfanilamide or penicillin exhibited some antifouling protection as evidenced by the fact that marine growth was present but was not heavily encrusted. The section of the cylinder which contained no coating was very heavily encrusted with barnacles and other marine life to a thickness of at least one inch. 
     In other experimental tests, tetracycline in the preferred form of oxytetracycline was employed as an antifouling agent in ordinary house paint and applied to a cylinder and tested as described above. The marine growth was inhibited very well at concentrations of about 500 mg.-1 g. of agent per quart of paint, but the inhibitory effect was not quite as good as that found when the basic paint composition was a quality marine bottom paint. 
     It has been found that the inhibitory effect can be maintained over long periods of time if the surface of the antifouling composition of this invention is lightly brushed or sponged every two or three months to wipe off any minor accumulation of slime or the like which attaches itself by electrostatic forces to the submerged surface. It has been estimated that when treated in this manner the surface is free of marine growth at least 50% longer, and in many instances at least 100% longer, than that achieved by any prior known antifouling paints. 
     It has been found that crude tetracycline is more effective in this invention as compared with the highly refined form that is suitable for human use. The crude form contains more of the natural by-products of the synthesis by microrganisms and has probably more chemical variants than the highly refined human form of tetracycline. The difference between these two materials is not presently known. 
     It has been determined that the following agents are usuable in the practice of the present invention, as alternatives to and in some cases additive to either tetracycline or one or more of each other: 
     (a) carbenicillin disodium 
     (b) ampicillin trihydrate 
     (c) cefoperazone sodium 
     (d) cefoperazone dihydrate 
     (e) doxycline hyclate 
     (f) carbenicillin 
     The maximum and minimum of the above-noted compounds is essentially the same minumum and maximum ranges as for tetracycline. 
     It has also been determined that in the case of doxycline hyclate and cefoperazone dihydrate, the activity is considerably stronger than tetracycline, from 2 to 5 times the overall strength as an antifoulant as compared to tetracycline. 
     Finally, the present invention contemplates the presence of the active particles as embedded in the coating or chemically linked in or to a vehicle. 
     While the invention has been described with respect to certain specific embodiments, it will be appreciated that many modifications and changes may be made by those skilled in the art without departing from the spirit of the invention. It is intended, therefore, by the appended claims to cover all such modifications and changes as fall within the true spririt and scope of the invention.
Marine coating for inhibiting the growth of marine life on surfaces in contact with seawater comprising a coating material having dispersed therein an effective amount of particles of a polycyclic naphthacenecarboxamide, and/or other active agents. A preferred agent is tetracycline. The principal use of the composition of this invention is to coat the hulls of boats and ships and other surfaces contacted by seawater.