GB2166143A - Anti-fouling paint - Google Patents

Abstract

A self-polishing anti-fouling paint comprises a biocide for marine organisms and a binder which is a linear synthetic film-forming polymer carrying a plurality of free carboxylic acid groups or carboxylic anhydride groups and having an equivalent weight of from 240 to 750. The binder is preferably an additional copolymer of one or more olefinically unsaturated acids or anhydrides.

Description

SPECIFICATION Anti-fouling paint This invention is concerned with self-polishing anti-fouling paints. An anti-fouling paint is used as a top coat paint on ships' hulls to inhibit the settlement and growth of marine organisms such as barnacles and algae, generally by release of a biocide for the marine organisms. A selfpolishing anti-fouling paint binder is gradually dissolved away from the hull surface, as the ship moves through sea water, in such a way that the paint surface at least preserves its initial smoothness and may become smoother.

Hitherto, self-polishing anti-fouling paints have used as binders linear polymers containing pendent side groups (hereinafter referred to as "Leaving Groups") which are liberated from the polymer by reaction with sea water, the residual polymer being sufficiently dispersible or soluble in sea water to be swept from the paint surface, exposing a fresh layer of the binder able to undergo a similar reaction with sea water. The gradual thinning of the paint film controls the release of a biocide active against fouling and present as a pigment in the paint or as a Leaving Group. Currently, the only commercially significant paints of this kind use binders which are copolymers of tributyltin methacrylate in which the Leaving Group is tributyltin, itself a biocide.

Examples of such paints are described in British Patent No. 1,457,590 and US Patent No.

4,191,579.

There have been proposals to replace the tributyltin Leaving Group of a self-polishing copolymer with a different Leaving Group, and, as the biocidal requirements of the paint may be satisfied by a biocidal pigment, it has been realised that it is not necessary that an alternative Leaving Group be biocidal. European Patent No. 69,559 and International Patent Publication WO 84/02915 make such proposals.

We have found a binder for a self-polishing anti-fouling paint requiring no Leaving Group.

According to the present invention a self-polishing anti-fouling paint comprising a biocide for marine organisms and a binder which is a linear synthetic film-forming polymer carrying a plurality of free carboxylic acid groups or carboxylic anhydride groups and having an equivalent weight of from 240 to 750, preferably from 240 to 700 (calculated as acid groups).

Anti-fouling paints based on rosin are well known and are described for example in the book "Marine Fouling and its Prevention" published by United States Naval Institute in 1952. Rosin is a carboxylic acid functional material but is not a linear polymer and cannot be used as the binder of a self-polishing paint. It erodes in service to form a brittle matrix of spent rosin which, although it may be gradually washed from the hull surface by sea water, increases in roughness over time. The paints of the present invention also have the advantage that the linear synthetic polymer forms a tougher and more coherent film.

The principle of known successful binders of self-polishing paints is that a substantially nonionic polymer having negligible solubility in sea water is changed by reaction with sea water into an ionic polymer able to dissolve or to be dispersed in sea water at the desired rate. The known binders accomplish this with the help of Leaving Groups. We believe that in the paints of the present invention the carboxylic acid or anhydride groups of the binder are substantially nonionic in the body of the paint film, but become carboxylate ions at the interface of the paint film and sea water. Surprisingly, the binders which characterise the paints according to the invention do not swell unduly in sea water. Polymer films from such binders immersed in sea water for up to 6 months regained their initial hardness when dried and showed no cracking or whitening.

A preferred binder for the paints of the present invention is an addition copolymer of one or more olefinically unsaturated acids or anhydrides, for example acrylic acid, methacrylic acid, maleic acid, maleic anhydride, fumaric acid, itaconic acid or itaconoic anhydride, vinylbenzoic acid (e.g. p-vinylbenzoic acid), 3-butenoic acid or beta-carboxyethyl acrylate or methacrylate, with at least one olefinically unsaturated comonomer. Copolymers of methacrylic acid or acrylic acid are preferred.

Examples of suitable comonomers include acrylic monomers, for example an acrylic or methacrylic ester, acrylamide or acrylonitrile, and vinyl monomers, for example vinyl acetate, vinyl butyrate, vinyl chloride, styrene, vinyl pyridine, N-vinyl pyrrolidone or vinylidene chloride. Binders which are terpolymers may be preferred, for example methyl methacrylate which tends to form a hard film can be used in conjunction with an acrylate such as ethyl acrylate or particularly an alkyl acrylate or 3 to 8 carbon atoms in the alkyl moiety such as butyl acrylate which helps to form a more flexible film. Substituted acrylate esters, for example hydroxyethyl acrylate or methacrylate, may be used as one of the comonomers.We have found that the use of styrene as a comonomer tends to retard the rate of dissolution of the paint in sea water; it may be preferred to include styrene as a comonomer in binders having a high proportion of acid or anhydride groups, for example having an equivalent weight of 240 to 440.

The preferred molecular weight for the copolymer binders for the paints of the present invention is 30,000-100,000, most preferably 40,000-90,000. This is higher than has generally been used for anti-fouling paints, for example organotin self-polishing paints, but these higher molecular weights have been found to give the best combination of resistance to swelling in water with self-polishing properties. The equivalent weight of the binder is most preferably in the range from 300 to 550. The proportion of units derived from an olefinically unsaturated acid or anhydride in the copolymer is generally in the range from about 10 to about 35 per cent by weight, preferably 13 to 30 per cent by weight, most preferably 15 to 25 per cent by weight, for acrylic or methacrylic acid.

Such a binder is preferably made by copolymerising the olefinically unsaturated acid or anhy - dride and the comonomer(s), preferably in solution in a solvent for the copolymer, using a free radical initiator. Preferred solvents are alcohols, for example butan-l-ol, pentan-l-ol or butan-2-ol, or mixtures thereof with up to an equal volume of an aromatic hydrocarbon such as xylene.

Ether alcohols and their esters can be used, for example methoxypropanol, methoxypropyl acetate, ethoxyethanol or ethoxyethyl acetate, as can esters such as butyl acetate and ketones such as methyl isobutyl ketone. Any of the conventional initiators can be used, for example an azo compound such as azobisisobutyronitrile or a peroxide.

An alternative binder for the paints of the present invention is a non-air-drying, carboxy functional linear polyester having an equivalent weight of from 240 to 750, preferably 300 to 550, for example a maleinised alkyd. A further alternative is a linear acid-functional epoxy resin, for example a reaction product of a diglycidyl ether such as the diglycidyl ether of bisphenol A with a dicarboxylic acid such as succinic or adipic acid in a molar ratio of at least 1:2.

The anti-fouling paint preferably contains a metalliferous pigment which is sparingly soluble in sea water or is a substantially water-insoluble pigment capable of reacting with sea water. The pigment is preferably a marine biocide, for example cuprous thiocyanate, cuprous oxide, zinc oxide, zinc ethylene bis(dithiocarbamate) or cuprous ethylene bis(dithiocarbamate). These pig ments produce water-soluble metal compounds on reaction with sea water so that the pigment particles do not survive at the paint surface. Preferably, the pigment has a solubility in sea water of from 1 to 10 parts per million by weight. The pigment has the effect of inducing the overall smoothing which the relatively moving sea water exerts on the paint film, minimising localised erosion and preferentially removing excrescences formed during application of the paint.

Cuprous thiocyanate and zinc ethylene bis(dithiocarbamate) are the preferred pigments for use in the present invention. The binders which characterise the paints of the present invention may be unstable in the presence of some sea-water-reactive pigments, for example zinc oxide or cuprous oxide. These pigments may be rendered insensitive to the binders by micro-encapsula tion in or coating with a polymer labile to sea water. For example, the pigments may be reacted with a dilute solution of a polycarboxylic acid copolymer such as one of the binders which characterise the paints of the present invention or with a pigment dispersant such as 'Sol sperse', and the reaction product then separated from the solution and incorporated with the binder in formulating the paint.Alternatively, the active sites on the surface of the pigment paticles may be rendered unreactive to the binder by treatment with a complexing agent such as an oxalate or a titanate ester or titanyl complex, for example titanium acetylacetonate.

The paint composition may additionally contain a pigment which is highly insoluble in sea water (solubility below 1 ppm by weight), such as titanium dioxide, ferric oxide or talc. Such highly insoluble pigments can be used in proportions up to 50 per cent by weight of the total pigment component of the paint. The highly insoluble pigment has the effect of retarding the dissolution of the paint. Alternatively, the paint can contain a hydrophobic organic retarder of the type defined in British Patent No. 1,457,590.

The biocide for marine organisms used in the paint of the present invention is preferably a metalliferous pigment as described above, particularly a copper compound such as cuprous thiocyanate or a dithiocarbamate such as zinc ethylene bis(dithiocarbamate) or cuprous ethylene bis(dithiocarbamate). The paint can contain an added biocide or biocides effective against algae, sea weed, barnacles and/or diatoms. Triorganotin salts and oxides such as triphenyltin fluoride, tributyltin fluoride, tributyltin dibromosuccinate, triphenyltin chloride, triphenyltin hydroxide and tributyltin oxide are effective marine biocides and can be used in paints according to the invention, although for environmental reasons it may be desired to avoid the use of triorganotin compounds. Examples of other biocides effective as marine antifouling agents are dithiocarbam ate derivatives such as 2-(N,N-dimethyl thiocarbamoylthio)-5-nitro thiazole or tetramethyl thiuram disulphide, substituted isothiazolones and dichloro diphenyl trichloroethane.

The invention is illustrated by the following Examples: Examples 1 to 6 Methacrylic acid, methyl methacrylate and butyl acrylate were copolymerised at 75"C for 4 hours in solution in butan-l-ol at 40 weight per cent solids using azobisisobutyronitrile initiator. A series of copolymers containing diferent acid levels was prepared. The apparent molecular weight of each copolymer was measured by gel permeation chromatography against polystyrene.

The Table shows the composition and molecular weight of the copolymers formed.

Percent by Weight in Copolymer Example Methacrylic Methyl Butyl Molecular Equiva No. Acid Methacrylate Acrylate Weight lent Weight 1 16.0 43.2 40.8 60000 538 2 12.0 47.4 40.6 80000 717 3 13.1 46.2 40.7 80000 657 4 18.7 40.3 41.0 80000 460 5 22.9 35.9 41.2 90000 376 6 24.7 33.9 41.4 80000 348 Films of pencil hardness HB could be cast from each of the polymer solutions formed.

Example 7 The polymer solution of Example 1 was blended with pigments and additional solvent to give a marine anti-fouling paint of the following composition: Per cent by Weight Polymer of Example 1 24.4 Butan-l-ol 53.6 Zinc ethylene bis(dithiocarbamate) 12.3 Talc 5.2 Titanium Dioxide 2.3 Structuring Agents 1.7 Phthalocyanine Pigment 0.4 The smoothing performance of this paint was tested using the apparatus described in British Patent No. 1,457,590 with reference to Figs. 7a and 7b. The paint of Example 7 was tested by comparison with a commercially successful organotin copolymer paint. The paint of Example 7 was seen to be gradually smoothed away by movement through sea water. Its rate of disoolution was 0.40 times that of the commercial organotin copolymer paint.

Examples 8 to 12 Paints were made up from the binders of Examples 2 to 6 in the same manner as in Example 7. The resulting paints were found to be gradually smoothed away in sea water, the rate of dissolution increasing with increasing acid content of the copolymer binder.

Claims (15)

1. A self-polishing anti-fouling paint comprising a biocide for marine organisms and a binder which is a linear synthetic film-forming polymer carrying a plurality of free carboxylic acid groups or carboxylic anhydride groups and having an equivalent weight of from 240 to 750 (calculated as acid groups).

2. A paint according to claim 1, wherein the binder polymer has an equivalent weight of from 240 to 700.

3. A paint according to claim 2, wherein the binder polymer has an equivalent weight of from 300 to 550.

4. A paint according to any of claims 1 to 3, wherein the binder is an addition copolymer of one or more olefinically unsaturated acids or anhydrides.

5. A paint according to claim 4, wherein the copolymer is a terpolymer containing methyl methacrylate and an alkyl acrylate having 3 to 8 carbon atoms in the alkyl group as comonomer units.

6. A paint according to claim 4, wherein the binder polymer has an equivalent weight of from 240 to 440 and includes styrene as a comonomer.

7. A paint according to any of claims 4 to 6, wherein the binder is an addition copolymer of acrylic acid or methacrylic acid.

8. A paint according to any of claims 4 to 7, wherein the molecular weight of the polymer binder is from 30,000 to 100,000.

9. A paint according to claim 1, wherein the binder polymer is a polymer substantially as described in any of the foregoing Examples 1 to 6.

10. A paint according to any of claims 1 to 9, wherein the biocide comprises a metalliferous pigment which is sparingly soluble in sea water or is a substantially water-insoluble metalliferous pigment capable of reaction with sea water.

11. A paint according to claim 10, wherein the pigment is cuprous thiocyanate.

12. A paint according to claim 10, wherein the pigment is zinc ethylene bis(dithiocarbamate).

13. A paint according to claim 10, wherein the pigment is cuprous oxide or zinc oxide encapsulated in or coated with a polymer labile to sea water.

14. A paint according to claim 1 and substantially as described in any of the foregoing Examples 7 to 12.

15. A process for coating a ship's hull comprising applying a paint according to any of claims 1 to 14 as a top coat paint to the hull.