This is a Continuation-in-Part of identi¬ cally titled application Serial No. 07/193,805 filed May 13, 1988.

Background of the Invention

1. Field of the Invention

The present invention is broadly concerned with a novel antifoulant coating for marine surfaces and a novel method for applying the coating to such surfaces. More particularly, it is concerned with a polyamide coating for boat hulls which in one em- bodiment preferably includes Nylon 11, carbon black and tributyltin fluoride for purposes of yielding a long lasting, marine growth resistant, environmen¬ tally safe coating for boat hulls. In other embodi¬ ments, the compositions of the invention advanta- geously includes a predominant amount of elemental copper or tin, together with a nylon terpolymer adhesive, an algicide and Nylon 11. The com¬ positions are applied to provide a marine growth- resistant coating to a boat hull by providing an electric arc, directing a gas stream through the arc thereby heating the gas stream, injecting the powdered composition into the heated gas stream at a location downstream from the arc so as to melt the powder without overheating the same, and then apply¬ ing the melted mixture to the surface to be coated. 2- Description of the Prior Art

Traditionally, high friction, abradable paints containing significant levels of antifoulants

have been applied to boat hulls to prevent marine growth thereon. Such paints sometimes contain in excess of 50% by weight of toxic antifoulants which leach to the surface of the paint, thereby retarding the formation of marine growths thereon. These conventional paints are also designed to gradually wear away as they are subjected to frictional forces encountered as the boat moves through water. This wearing away due to abrasion facilitates the gradual removal of marine growth from the boat and thereby continuously provides a new and growth-free paint surface on the boat hull.

Typical paint application technigues involve compressed air spraying of liguid paints onto prepared boat hull surfaces. Preparation usually involves cleaning of the hull surfaces, and in the case of surfaces which are being repainted, sanding or sandblasting is usually reguired in order to remove the old paint prior to repainting. These traditional paints and paint appli¬ cation technigues have a number of problems or shortcomings. Abradable paints containing high levels of toxic, tin-based antifoulants result in substantial environmental pollution due to the direct and rapid release of large guantities of these antifoulants into the water. The environment¬ al impact of these paints is intensified by the loss of tin containing paint particles into the water as a result of abrasion. Accumulation of tin in shal¬ low waters can and does kill and deform fish and other marine life and ^' may ultimately pose a hazard to human consumers of seafood. Furthermore, tin- based antifoulants have been used on aluminum boats to the exclusion of copper based antifoulants due in

part to a corrosive reaction between copper and aluminum which can cause extensive damage to alumi¬ num surfaces. Additionally, boats coated with conventional paints must be occasionally blasted with high pressure water spray, which adversely affects the adherence of the paint to the hull, in order to remove built up deposits such as scale. Another shortcoming of many of these paints is that they are relatively shortlived, guickly becoming ineffective due to their tendency to rapidly leach their antifoulants and their propensity to experi¬ ence substantial wear during use of the boat. As a conseguence, conventional paints typically require relatively frequent removal and replacement. Remov- al of the old paint by techniques such as sanding and sandblasting can create substantial quantities of toxic paint dust which when inhaled can cause serious illness and when washed into surrounding waters can pollute the marine environment. Further- more, conventional spraying techniques typically result in substantial paint overspray and misting which likewise can cause serious illness if inhaled.

Consequently, a need exists for improve¬ ment in antifoulant, protective coatings for marine surfaces which will result in reduced environmental pollution while at the same time providing a low maintenance, wear-resistant, protective coating which resists marine growth.

Summary of the Invention

The problems outlined above are in large measure solved by the present invention which pro¬ vides an improved, long-lived, non-abrading, dense coating for marine surfaces which is particularly designed to be applied to marine surfaces such as

boat hulls, submarine hulls, and propellers, for purposes of preventing marine growth from forming thereon while minimizing environmental pollution and health hazards resulting from the coating. Broadly speaking, in one aspect of the invention, the coat¬ ing composition comprises respective amounts of a polyamide in the nature of Nylon 10 or 11, an anti¬ foulant, and preferably also an inorganic porous carrier for the antifoulant. These ingredients are selected and mixed so that the composition, when applied to a marine surface, will provide a marine growth resistant coating which is low friction, long lasting, wear-resistant and relatively environmen¬ tally safe. Advantageously, this resultant pro- tective coating has an antifoulant release rate of less than 5 micrograms per square millimeter per day so as to minimize environmental pollution while at the same time maintaining a protective coating against marine growth. Therefore, the composition of the invention can be used to good effect in protecting boat hulls while providing a surface coating which experiences reduced wear, extended life, and reduced environmental impact.

Additionally, the present invention con¬ templates the application of tin- or copper-based antifoulants to aluminum boats by first coating the hull of the boat with a dense, protective precoat preferably about five to ten thousandths of an inch thick of Nylon 10 or Nylon 11 and then applying the antifoulant coating of the present invention thereon.

In particularly preferred forms, the composition of the invention includes Nylon 11, carbon black, and tributyltin fluoride antifoulant.

However, other forms of nylon (e.g. Nylon 10) as

well as other antifoulants (e.g., cuprous oxide and bis-tributyltin adipate, C ₃₀ H _g2 O ₄ Sn) find utility in the invention. Porous inorganic powered substitutes of carbon black which serve as a carrier for the antifoulant (e.g., perlite, zinc oxide, various paint clays and pigments) also can be used in the invention. In yet another form, the composition of the invention may include a portion of Si0 ₂ glass spheres. The glass spheres do not melt when com¬ bined and sprayed with the polyamide coating as disclosed herein and imbed in the antifoulant coating to increase the strength and abrasion resis¬ tance of the antifoulant coating hereof.

In other preferred compositions, elemental tin or copper in particulate form (49 microns or less) is employed in substantial amounts ranging from about 72% by weight up to an amount which would render the composition effectively electrically conductive, in combination with a polyamide such as Nylon 10, Nylon 11 or mixtures thereof. In prac¬ tice, the particulate, elemental tin or copper is provided at a level of from about 72-82% by weight. Such compositions further normally include from about 4-8% by weight of a nylon terpolymer adhesive, about 4-8% algicide, with the balance being a poly¬ amide such as Nylon 11.

In coating application techniques, the powdered compositions, of the invention may be injected into a high velocity, heated, plasma gas stream which converts the powdered compositions to a molten state and imparts a high velocity thereto. The molten co postions and associated gas are then directed into substantially one direction thereby reducing any tendency of the molten compositions to overspray or mist. The molten compositions are di-

rected at a prepared marine surface, whereupon they cool and bond with the marine surface. High velo¬ city impact of the molten compositions onto the marine surface increases the bond strength there¬ between. The resultant bonded polyamide coating is wear-resistant, contains relatively small amounts of antifoulant and leaches the antifoulant relatively slowly, but at a rate sufficient to retard marine growth thereon. Prior techniques and structures for the application of polyamides or epoxy resins by plasma spray are set out in U.S. Patents No. 3,851,140 and 4,049,842.

In addition to the aforementioned advan¬ tages, the coatings hereof have relatively low coefficients of friction and as a result will reduce the noise, drag and energy needs of the boat as it moves through the water. Subsequent coatings can be applied onto the existing coatings without a sub¬ stantial sacrifice in the bond strength of the resultant coatings. Thus, new coatings can be applied without necessitating the removal of pre¬ vious, exhausted coatings. One additional aspect of the present invention is that the total antifoulant content of the coatings can be reduced by applying antifoulant-free Nylon 11 undercoats prior to apply¬ ing an outer antifoulant-containing final coat.

In addition, the present invention contem¬ plates providing a skid-proof coating by combining and jointly plasma spraying a friction enhancing agent such as particles of aluminum oxide along with Nylon 11 and/or the antifoulant composition of the present invention.

The protective coatings disclosed herein may be applied using the apparatus shown in my U.S.

Application Serial No. 07/193,739 filed May 13, 1988, and such application entitled Apparatus and Method for Applying Plasma Flame Sprayed Polymers, which is incorporated herein by reference.

Description of the Preferred Embodiments

One preferred polyamide coating in accord¬ ance with the present invention includes about 82% by weight of Nylon 11, about 6% by weight of carbon black, and about 12% by weight of tributyltin fluoride. This composition is initially in the form of a powdered mixture and is subsequently melted for application to a marine surface whereupon it cools and forms a dense, impact resistant coating. In its broader aspects, this embodiment of the invention comprehends coating compositions having from about 62.5 to 82% by weight of polyamide selected from the group consisting of Nylon 10, Nylon 11 and combinations thereof, and more prefer- ably from about 70 to 82% by weight thereof; from about 25 to 60% by weight antifoulant such as tin or

I" copper-based antifoulant materials, and preferably from about 25 to 37% by weight thereof; and from about 6 to 12% by weight porous inorganic carrier powder selected from the group consisting of carbon black, zinc oxide and perlite, and more preferably from about 8 to 12% thereof.

Nylon 11, also known as poly.imino (1- oxo-1, 11-undecanediyl) ] , is a polyamide having the chemical formula H[HN-(CH_ ) _{χ 0} -CO] _n OH where n is approximately 50. The present invention also con¬ templates using various other forms of poly.imino (1-oxo-l, 11-undecanediyl) ] including those where n is from 25 to 100. Nylon 11 can be obtained by the

self condensation of w-amino undecanoic acid or can be obtained as a condensate of hexa ethylene diamine and w-amino undecanoic acid. This polyamide comes in various grades of powder, has a melting point of 184 to 186°C, a bulk density of approximately .4 to .5 and a true density at 20°C of approximately 1.04 to 1.12. Nylon 11 is available commercially, and in actual practice may be obtained from Atochem, Inc. of Glenrock, New Jersey, under the trade name Ril- san. Nylon 10 is a powdered, polyamide having the chemical formula H[HN-(CH ₂ ) ₉ -CO] _n OH where n is preferably from about 25 to 100 and more preferably about 50. Nylon 10 is also available commercially. Tributyltin fluoride is an antifoulant having a chemical formula (C ₄ H ₉ ) ₃ SnF. Tributyltin fluoride has a specific gravity of 1.27, and a molecular weight of 309.39, a solublity in water of .029 grams per liter and at room temperature is a white powder having a characteristic odor. Tribu- tyltin fluoride is available commercially, and in actual practice may be obtained from M & T Chemi¬ cals, Inc. of Rahway, New Jersey.

Another suitable antifoulant contemplated by the present invention is cuprous oxide having the formula Cu _z O. Cuprous oxide, also known as copper oxide red, is in the form of reddish brown, octahe¬ dral crystals which are insoluble in water, have a density of 5.75 to 6.09, and a melting point of 1,235°C. Cuprous oxide is available commercially, and in actual practice may be obtained from American Chemet Corporation.

Carbon black is a porous inorganic powder and in the present invention is used to increase the amount of antifoulant which can be carried by the

polyamide and to increase the rate of leaching of the antifoulant from the polyamide. Carbon black, due to its porosity, serves to both carry the anti¬ foulant and increase the porosity of the polyamide. Carbon black also serves to prevent powdered tribu¬ tyltin flouride from "balling up" thereby enhancing the flowability of the premelt powdered mixture. A usable form of carbon black is gas black which is a fluffy black pigment produced by incomplete com¬ bustion of natural gas. Another usable form of carbon black is furnace black which is a finely divided form of carbon made by burning vapor and heavy oil fraction in a furnace at 50% of the air required to complete combustion. Other forms of carbon black also find utility. The preferred carbon black product is purchased from Union Car¬ bide. Another suitable porous inorganic carrier is perlite which is known as the eutectic between ferrite and cemetite and has a laminar structure resembling mother-of-pearl, barely resolvable under the finest microscopes, and occurs in steel as a result of the transformation of austenite into aggregations of ferrite and cementite. Perlite is commercially available, and in actual practice may be obtained from Borget Brothers, Santa Monica, California.

Zinc oxide (ZnO) is another carrier con¬ templated by the present invention. Zinc oxide is a coarse white or grayish powder which is odorless and has a density of 5.47 g/cm ³ and a melting point of 1975°C. Zinc oxide is commercially available and in actual practice may be obtained from TRI-ESS Sci¬ ences, Burbank, California.

The composition hereof may include a portion of hollow, Si0 ₂ glass spheres of a con¬ trolled particle size and density. The spheres are preferably 0.60 grams per cubic centimeter in den¬ sity and average 65 to 75 microns in diameter. The hollow spheres act as microscopic bearings with a minimum ratio of surface area to volume, thereby reducing viscous drag, resisting abrasion to the surface and providing improved flow properties. The hollow spheres comprise .20% by volume of the poly¬ amide coating, and may be provided with a copper coating when, for example, the composition hereof includes a portion of cuprous oxide or other copper- based antifoulant. The glass spheres are available commercially, and in actual practice may be obtained from the 3-M company of St. Paul, Minnesota under the name "Glass Bubbles.", either with or without a copper coating.

In preferred production procedures, the powdered compositions in accordance with the inven¬ tion are prepared by thoroughly mixing respective powdered amounts of the polyamide, the porous inor¬ ganic carrier, and the antifoulant. Preferably, the powder is prepared by mixing respective amounts of agglomerates of carbon black and tribultyltin flour¬ ide in a Hobart Blender at 3600 RPM for 2 minutes, homogenizing this mixture into powder of a size of approximately 325 Mesh (120 microns), and then mixing respective amounts of these pellets, Nylon 11 and glass spheres.

In another aspect of the invention, compo- sitions are provided including relatively large quantities of antifoulant such as elemental tin or copper or cuprous oxide, and a polyamide such as

Nylon 10, 11, or 12-12. Such compositions also

normally include minor amounts of adhesive and algicide. Thus, a preferred composition may include from about 54%-82% by weight of elemental tin or copper (most preferably about 82% by weight) of a particle size preferably less than 49 microns, about 4-8% of an adhesive (advantageously about 4% by weight of a nylon terpolymer/caprolacta adhesive sold under the designation "M548" by Atochem, Inc. of Birdsboro, Pennsylvania) of a particle size 0 preferably less than 150 microns, from about 4-8% of an algicide (preferably about 4% by weight of a material sold under the designation "VANCIDE 89" by the R.T. Vanderbilt Company, Inc. of Norwalk, Connecticut, which is N-trichloromethylthio-4- 5 cyclohexene-1,2-dicarboximide) of a particle size preferably of 5 microns or less, and with the balance being Nylon 11 or alternatively, Nylon 12-12 manufactured by E.I. DuPont de Nemours and Company of Wilmington, Delaware. The Nylon 11 or other o polyamide preferably is of a particle size of 150 microns or less. This preferred composition has been found to be extremely effective, and is par¬ ticularly advantageous from an ecological standpoint in that it has a leach rate of less than 1 microgram of antifoulant per square millimeter per day. 5 Furthermore, use of elemental tin or copper avoids the handling problems inherent in the use of tri¬ butyltin antifoulants_

Other compositions within the ambit of the invention may include from about 40 to 70% by weight cuprous oxide (preferably about 52% by weight, about .4-8% by weight of adhesive (preferably about 4% by weight of nylon terpolymer adhesive), about 8% by weight of the "VANCIDE 89" material, and with the

balance being either Nylon 10, Nylon 11, Nylon 12-12, or mixtures thereof.

Here again, the above-described compo¬ sitions are prepared by mixing the metallic anti¬ foulant, adhesive and algicide in a conventional blender, such as a Hobart blender. Such powdered particles are then blended for two to three minutes at 3600 r.p.m.'s with the polyamide to provide the final coating composition which, after blending, has preferably a mesh size of about 44 microns.

Preferred techniques for applying the coating compositions of the present invention include the steps of providing a high velocity flow (i.e., about Mach I or above) of a gas such as pure argon; passing the gas transversely through an elongated high wattage electric arc for heating the gas and converting a portion thereof to the plasma state; injecting the powdered coating composition into the gas downstream from the arc for melting the powder without overheating the powder; directing the flow of the composition and associated gas into substantially one direction for minimizing overspray and misting of the composition; and spraying said melted composition onto a surface to be coated. More preferably, the powdered composition is inject¬ ed into the gas stream in a downstream direction and at an angle of from about 12 to 20° to the direction of flow of the stream; and most preferably the powdered composition is injected in a downstream direction and at an angle of about 18° to that of the direction of travel of the gas stream so as to minimize vortex formation within the stream and minimize the over spray associated with vortex formation. Also more preferably, the powder is injected at a distance of from about 6 to 10 inches

downstream from the arc (the arc being defined as a point of energy transfer between an anode and a cathode) so as to minimize overheating of the com¬ position and so as to ensure that the composition reaches maximum velocity for a corresponding maximum bond strength with the surface to be coated; and more preferably, injecting the composition into the gas stream at a location of from about 5 to 8 inches downstream from the arc so as to achieve the proper molten state of the composition and a particle velocity favoring interatomic bonding of the compos¬ ition with the surface to be coated.

If injection of the powdered composition is made either through a high wattage arc or closely adjacent thereto, the composition will be overheated and rendered useless. If a lower wattage arc is employed so as to generate a temperature low enough to permit injection of the powder either through the arc or adjacent thereto, then the application rate permitted by the arc will be so low as to make large scale application economically infeasible. Thus, injection of the powdered composition substantially downstream from the arc is advantageous to achieve an economically feasible, high volumetric rate application technique. Also, injection of the powder downstream from the arc permits increased arc temperature, which in turn permits adequate heating of increased flows of gas thereby permitting ade¬ quate melting and particle velocity for increased powder flow rates.

Plasma spraying has been used in the past in the context of spraying either ceramics or pow¬ dered metals but such materials do not have the temperature limitations of polyamide coatings. The present invention represents applying powdered

polyamide compositions involving injection of the powder at a sufficient distance downstream from the arc so as to achieve for the first time, high volu¬ metric application rates of polyamide coatings. The arc used in the method of the present invention has a preferred power level of 20 to 40 kilowatts and an associated gas temperature at the arc of approximately 12,000 to 30,000°F. The gas is then cooled so that by the time the gas has reached 0 the inlet for the powdered composition, the temper¬ ature of the gas has dropped down to approximately 250 to 800°F while travelling at a speed of 5,000 to 7,000 feet per second. Gases useful in this inven¬ tion include N ₂ , H ₂ , He, Ar and combinations there- 5 of. The coatings made from the composition of the present invention when applied using the application techniques of the present invention provide coatings having application rates, densities and bond strengths substantially greater than that of coat- o ings applied by conventional polyamide application techniques such as fluidized bed dipping, acetylene flame spraying and electrostatic spraying.

The plasma spray method of the present invention further involves vacuuming toxic fumes from a periphery of the stream adjacent the surface 5 to be sprayed for minimizing the escape of toxic fumes generated during heating of the composition, into the atmosphere. Vacuumed gases are oil filter¬ ed to remove the toxic gas fumes and organic acid vapors. The vacuum preferably pulls at a rate of at 0 least 10 inches of water at 85 and preferably 360 .cubic feet per minute.

In coating a marine surface such as an aluminum boat hull, the hull may be treated in a stepwise fashion. That is, the hull may first be 5

precoated as set forth hereinabove by injecting a powdered polyamide selected from the group of Nylon 10 and Nylon 11 into the heated gas stream.to melt the polyamide. The polyamide thus melted is then applied to the hull by directing the heated gas stream toward the surface of the hull. The poly¬ amide coating thus applied is permitted to melt to form a precoat. The polyamide precoating is then sprayed with an antifoulant coating mixture including an antifoulant and a polyamide such as Nylon 10 or Nylon 11. The antifoulant coating mixture is similarly injected into a gas stream which has passed through an electric arc thereby melting the antifoulant mixture. The melted anti- foulant coating mixture is then applied over the precoat and allowed to cool therein, producing a final antifoulant coating on the aluminum hull.

A skid resistant coating may also be applied according to the method disclosed herein. The coating includes respective amounts of Nylon 10 or Nylon 11, or combinations thereof, and a portion of solid particles of aluminum oxide, which are not melted during the injection of the compositions in the heated gas stream.

The following example sets the preferred preparation of a powdered composition in accordance with the present invention, together with typical steps involved in application of the coating.

EXAMPLE

A premelt powdered coating composition is prepared by mixing 6 pounds of carbon black with 12 pounds of tributyltin: fluoride in a Hobart Blender at 3600 RPM for 2 minutes. This serves to blend and

homogenize the mixture into powdered particles of a size of approximately 325 mesh (120 microns). Such powdered particles are then mixed with 82 pounds of powdered Nylon 11 for two to three minutes at about 3,600 r.p.m.'s.

A directed stream of pure argon gas, at a flow rate of 1.42 cubic feet per minute (CFM) at 50 psi, is passed transversely through an electric arc having a power level of 12 kilowatts. The arc substantially heats the gas and causes some of the gas to be converted to the plasma state. This heated stream is then cooled as it moves away from the arc. The powdered composition mixture is then injected by use of a pressurized carrier gas into the heated gas stream at a location of about 8.5 inches downstream from the arc and in a downstream direction and at an angle of about 18° to that of the general flow of the gas stream. The mixture and the heated gas then combine to properly melt the mixture and impart a substantial velocity onto the melted mixture. The gas and mixture then are passed further downstream together in a substantially uniform direction so as to minimize overspray and are then sprayed directly onto a marine surface so as to form a film coating of the composition there¬ on. The film coating is then allowed to cool and bond with the surface.