GB2337950A

GB2337950A - Prevention of marine fouling

Info

Publication number: GB2337950A
Application number: GB9812174A
Authority: GB
Inventors: Paul James Dawe Mason; Jane Benson; Adrian James Lee; David Neil Williams
Original assignee: Courtaulds PLC; Akzo Nobel UK PLC
Current assignee: Akzo Nobel UK PLC
Priority date: 1998-06-05
Filing date: 1998-06-05
Publication date: 1999-12-08
Also published as: GB9812174D0

Abstract

A coating composition applied as a top coat paint for protecting an underwater surface against fouling by aquatic organisms contains a polysaccharide ester as a film-forming binder. The polysaccharide ester is dissolved in a solvent which is a non-solvent for the undercoat under the conditions of application. The coating composition preferably contains a catalyst which accelerates dissolution of the polysaccharide ester when a film of the polysaccharide ester is submerged in water.

Description

2337950 Prevention of Marine Fo-ulin This invention relates to coating

compositions and processes for protecting an underwater surface (that is, a surface which will be underwater in use) from fouling by aquatic organisms such as algae and barnacles. Examples of such surfaces are boat hulls, oil drilling and production rigs, the cooling water inlets and outlets of power stations and fish farming equipment.

The most common form of antifouling coatings are those containing a biocide for marine organisms which is gradually released from the coating; the most successful of these are of the,eroding" or "self -polishing" type in which the paint matrix is gradually hydrolysed and washed away at the surface as the biocide is released. There have been suggestions for coatings which resist fouling without release of a biocide. The most successful of these have been based on silicones, particularly silicone rubber, as described in GB-A-1307001, GB-A-1470465 and US-A-5663215.

N. A. Ghanem et al examined the fouling resistance of cellulose acetate and of an organosiloxane resin as described in J. Coatings Technology, Vol. 54, January 1982, at pages 8388, and concluded that they showed marine fouling resistance but that the period of protection did not exceed 40 days.

US-A-3990381 describes the application as a coating of hydrophilic polymers such as hydrophilic cellulose esters and cellulose ethers having encapsulated therein antifouling agents and/or pigments to underwater portions of a marinestructure to reduce drag.

GB-A-1576999 describes an antifouling paint including at least one toxic substance incorporated into a discontinuous solid matrix which is insoluble in sea water and dispersed in the paint. The matrix is at least partially formed from a substance, for example a gel of regenerated cellulose or coagulated protein, which can be dissolved in sea water under the action of enzymes liberated by bacteria and/or other 5 marine organisms.

A process according to the present invention for protecting an underwater surface against fouling by aquatic organisms comprises applying a top coat paint over an undercoat on the underwater surface and is characterised in that the top coat paint contains as film-forming binder a solution of a polysaccharide ester in a solvent which is a non-solvent for the undercoat under the conditions of application.

we have found according to the invention that coatings of polysaccharide ester, for example cellulose ester, resist fouling by aquatic organisms but that fouling resistance is greatly dependent on formation of a smooth coating film. Antifouling coatings are generally applied over an undercoat, usually an anticorrosive primer based for example on a cured epoxy resin or polyurethane or on a vinyl polymer which can be thermoplastic, for example a vinyl chloride/vinyl acetate copolymer, or thermosetting. Solvents used for cellulose esters in other applications, particularly solvents for cellulose acetate, are usually aggressive solvents such as acetone or methylene chloride which can partially dissolve even cured undercoats. This leads to cracking or distortion and the cellulose ester top coat loses its resistance to marine fouling.

The polysaccharide ester can for example Pe at least- partially a cellulose ester of a carboxylic acid having at least 3 carbon atoms, particularly an acid having 3 to 8 carbon atoms. The polysaccharide ester can for example be cellulose propionate, cellulose butyrate, cellulose acetate propionate, cellulose acetate butyrate, cellulose acetate phthalate, cellulose acetate valerate or cellulose propionate 3 valerate. Such a cellulose ester preferably has a degree of substitution of at least 1.2 or 1.5, and most preferably at least 2.4, carboxylate ester groups per anhydroglucose unit, up to 3 carboxylate ester groups per anhydroglucose unit. At least half of the ester groups can for example be ester groups of a carboxylic acid having at least 3 carbon atoms. Examples of mixed esters are cellulose acetate propionate and cellulose acetate butyrate having 0. 05 to 1.5 acetate groups, 0.5 or 1.0 up to 2.5 propionate or butyrate groups and 0 to 2.0 hydroxyl groups per anhydroglucose unit. Esters having a total degree of substitution of at least 2.4 carboxylate ester groups per anhydroglucose unit may be particularly preferred. Such cellulose higher carboxylate esters and mixed esters are soluble in a wide range of solvents which are regarded as suitable for use in paints and which do not significantly dissolve a cured undercoat. We have also found that the cellulose higher carboxylate esters and mixed esters may give better resistance to marine fouling than cellulose acetate. The cellulose ester top coat can for example be applied from solution in a ketone containing at least 4 (preferably at least 6) carbon atoms such as methyl isobutyl ketone or methyl isoamyl ketone, an ester containing at least 4 carbon atoms such as butyl acetate or methoxypropyl acetate, an alcohol or ether-alcohol having at least 4 carbon atoms such as n-butanol or 2-methoxypropanol, or a mixture of said ketone, ester, alcohol or ether-alcohol with an aromatic hydrocarbon such as xylene, toluene or trimethylbenzene. The coating can alternatively be applied from an aqueous composition, for example free carboxylic acid groups such as those in cellulose acetate phthalate can be solubilised by fugitive amine groups, or the coating can be an emulsion containing a coalescing cosolvent.

The polysaccharide ester can alternatively be cellulose acetate which can for example be dissolved in an ester of a hydroxycarboxylic acid such as ethyl lactate or in a hydroxyketone such as diacetone alcohol (2-hydroxy-2methylpentan-4-one) or in a diketone such as 2,5-hexanedione.

- 4 The cellulose acetate preferably has a degree of substitution of 1.3 to 2. 9, most preferably 1.9 to 2.3, acetate groups per anhydroglucose unit. The cellulose acetate is preferably prepared by full acetylation of cellulose to a degree of substitution of about 3 followed by partial hydrolysis, although controlled partial acetylation direct to a degree of substitution of 1.3 to 2.9 can be used. Other polysaccharide esters can be prepared similarly. Alternatively, an ester with a degree of substitution less than 3 can be prepared directly by reaction of the polysaccharide with the required amount of carboxylic acid chloride.

The cellulose ester may have a molecular weight of 60000150000 (measured by gel permeation chromatography) which is usual for cellulose esters for film or moulding applications, but it preferably has a lower molecular weight. Most preferably, the weight average molecular weight RQ of the cellulose ester is below 50000. The MP is usually above 5000 and preferably above 10000 and can for example be in the range 20000 to 30000.

The polysaccharide ester can alternatively be an ester of a different polysaccharide such as starch or amylose. For example, it can be starch acetate, starch hexanoate, amylose hexanoate, starch butyrate, starch palmitate or amylose palmitate. Blends of a starch carboxylate with a cellulose ester such as cellulose acetate may be advantageous.

We believe that the polysaccharide ester resists fouling because it is gradually hydrolysed by the water in which it is immersed, especially if this is sea water, and it is also gradually degraded by bacterial attack, particularly by the aquatic bacteria which colonise an underwater surface before the settlement of algae and barnacles. As the degree of substitution of the polysaccharide ester is reduced by hydrolysis, it gradually becomes more soluble in sea water; cellulose acetate for example is readily water- soluble at a degree of substitution of about 1.0. This leads to the polysaccharide ester being gradually washed away at the surface of the coating, similarly to the matrix of "eroding" antifoulings, so that a new paint surface is continuously revealed. We have found that coatings based on polysaccharide esters have the unusual advantage that they, unlike known commercial, eroding" antifoulings, tend to hydrolyse and/or degrade faster in static conditions, so that they resist fouling settlement when a ship is stationary.

whilst the polysaccharide ester generally hydrolyses and/or biodegrades at a sufficient rate to show resistance to marine fouling, we have found that hydrolysis can be catalysed and that the resulting faster hydrolysis may lead to improved resistance to fouling. Thus, according to another aspect of the invention a coating composition for protecting an underwater surface against fouling by aquatic organisms is characterised in that the coating composition contains a polysaccharide ester as film- forming binder and also contains a catalyst which accelerates dissolution of the polysaccharide ester when a film of the polysaccharide ester is submerged in water, for example in sea water. Accelerated dissolution can lead to improved fouling resistance by continuously revealing a fresh paint surface and by aiding biocide release, particularly in paints having a low concentration of biocide.

The polysaccharide ester can for example be cellulose acetate, a cellulose higher carboxylate ester or a cellulose mixed ester as described above.

One example of a preferred catalyst effective in accelerating hydrolysis of cellulose esters in sea water is a soluble salt of a polyvalent, preferably divalent, metal- The polyvalent metal can for example be zinc, magnesium, manganese, iron, cobalt, nickel, tin, calcium, barium, bismuth or aluminium. The salt can be a halide, for example chloride, or nitrate, sulphate or carboxylate, for example acetate. We have found that acetate salts are particularly effective in promoting hydrolysis at the outer surface of the coating film 6 which contacts the water rather than throughout the bulk of the film, which might weaken the coating film. Examples of particularly effective salts are magnesium acetate and zinc acetate. Stannous chloride is also an effective catalyst.

Alternative types of catalysts which are effective are metal oxides, particularly oxides of Groups IV B or V B metals such as germanium oxide or antimony trioxide.

A further type of catalyst effective in promoting cellulose ester hydrolysis is an acidic organic ester, particularly a partial ester of a strong acid such as sulphuric or phosphoric acid. Dialkyl hydrogen phosphates, for example, preferably having 1 to 12 carbon atoms in the alkyl group such as bis (2-ethylhexyl) hydrogen phosphate, are effective in catalysing hydrolysis of cellulose acetate.

is A further alternative type of catalyst is an enzyme promoting hydrolysis and/or chain reduction of a polysaccharide ester, for example an esterase or cellulase.

The concentration of catalyst in the paint can in general be up to 20 or 30% by weight based on the polysaccharide ester, for example from 0.2 to 1.0% up to 10% by weight.

The top coat paint based on a polysaccharide ester may be used with or without a biocide for aquatic organisms. This will depend partially on the location in which the painted surface is to be used and the severity of fouling to be expected. The polysaccharide ester top coat paint generally has sufficient fouling resistance to be used without biocide in cold waters such as around the Baltic, where the fouling" challenge is less severe. In these circumstances the polysaccharide ester top coat paint can be applied as a clear coat or can contain non-biocidal pigments such as titanium dioxide or phthalocyanine pigments. Such top coats, particularly cellulose ester clear top coats, have the advantage of improved mechanical properties such as tear resistance and abrasion resistance compared to roomtemperature-vulcanised silicone rubbers, which have been the most widely used non-toxic aquatic f ouling- resistant coatings used hitherto.

For use in warmer waters, particularly in or near the tropics, it may be preferred to use a polysaccharide ester top coat paint comprising a biocide for aquatic organisms. Such paint usually contains a pigment; the biocide may itself be pigment or may be an organic biocide dissolved or dispersed in the paint at a molecular level. The pigment and/or biocide, if used, can be blended with the polysaccharide ester and solvent using conventional paint blending techniques. If it is pigmented, the coating composition preferably has a pigment volume concentration of, for example, 5 to 55%, most preferably 15 to 40%. A biocidal pigment preferably comprises at least one sparingly soluble metalliferous pigment having a solubility in seawater of from 0.5 to 10 parts per million by weight. Examples of such pigments which are also aquatic biocides include copper or zinc compounds, such as cuprous oxide, cuprous thiocyanate, cuprous sulphate, zinc ethylene bis (dithiocarbamate), zinc dimethyl dithiocarbamate, zinc pyrithione, copper pyrithione, zinc diethyl dithiocarbamate, copper resinate or cuprous ethylene bis(dithiccarbamate) and certain other sparingly seawater- soluble metalliferous pigments, for example manganese ethylene bis (dithiocarbamate). Copper metal can be present as an aquatic biocide, for example in flake or powder form. The antifouling coating composition can contain a non- metalliferous biocide for marine organisms, for example tetramethyl thiuram disulphide, methylene bis(thiocyanate), captan, pyridinium triphenylboron, a substituted isothiazolone such as 4,5-dichloro-2-N-octyl-4isothiazolin-3-one, 2methylthio-4-t.butylamino-6cyclopropylamino-s-triazine, N-3,4dichlorophenyl-W,N1dimethyl-urea ("Diuron") 2- (thiocyanomethylthio) benzothiazole, 2,4,5,6-tetrachloroisophthalonitrile or 2,3,5,6-tetrachloro-4(methyl sulphonyl) pyridine. Such a non-metalliferous biocide can be used in addition to a sparingly soluble copper or zinc compound, or one or more non-metalliferous biocides can be used as the only biocide of the coating in a metal-free or pigment-free antifouling coating.

The coating composition can additionally contain one or more plasticisers and/or auxiliary film-forming agents, for example an ester plasticiser such as a polyol carboxylate ester, for example a glycerol tricarboxylate such as glycerol triacetate (triacetin) or a glycol dicarboxylate such as triethylene glycol diacetate or dipropylene glycol diacetate or a phthalate diester or a phosphate triester. Plasticisers which are themselves biodegradable or gradually dissolve in sea water are preferred since they do not interfere with the eroding of the paint matrix. Triacetin may dissolve in sea water too rapidly but the glycol carboxylates, or a mixture thereof with triacetin, can be used as plasticisers. Polycaprolactone is another preferred plasticiser and co- f ilmformer which is biodegradable and thereby is gradually eroded in sea water. Plasticisers and/or auxiliary film-forming agents, if present, are generally used at up to 30- 0 by volume based on the polysaccharide ester, preferably 5-20% by volume. The coating composition can additionally contain a thickener, for example a thixotrope such as silica or bentonite.

The invention is illustrated by the following Examples.

Example 1

A 25% by weight solution of cellulose acetate propionate (CAP) having a degree of substitution DS of 2.4 propionate and 0. 18 acetate groups per anhydroglucose unit in methyl isobutyl ketone (MIBK) was coated by brush over an epoxy primer-coated:

plywood test panel. It formed a hard, smooth, glossy coating.

The coated panel was immersed in sea water off the South Coast of England for a summer fouling season.

9 - Example 2

The procedure of Example 1 was repeated employing a cellulose acetate butyrate (CAB) (1.75 DS butyrate, 1.ODS acetate) in place of the CAP.

Ex@.pRle- 3 The procedure of Example 1 was repeated employing a cellulose acetate phthalate (2.4 DS acetate, 0.36 DS phthalate) at 10-6o by weight in methoxypropanol in place of the CAP solution.

All of the coatings of Examples 1 to 3 showed only slight fouling at the end of the Summer, compared to a panel coated with only a primer, which was heavily fouled by sea weed, barnacles and hydroids, showing that the coatings of Examples 1 to 3 were able to discourage barnacle, hydroid and algal growth upon immersion in sea water. Additionally, they could be cleaned with relative ease by sweeping with a soft brush, unlike the primer control which required scraping with a hard implement.

Examle 4 CAP (2.50 DS propionate, 0.07 DS acetate) was applied as a 25% by weight solution in xylene/methoxypropanol 50:50 by volume by brush over epoxy primer-coated plywood.

Exa=le 5 Cellulose valerate propionate (1.07 DS propionate, 1.65. 25 DS valerate) was applied as a 25% by weight solution in xylene/methoxypropanol 50:50 by volume by brush over a vinyl primer-coated plywood panel.

l Example 6

Starch hexanoate was applied as a 20 by weight solution in trimethylbenzenes by brush over vinyl primer-coated plywood.

ExamT)1e 7 Starch palmitate was applied as a 10% by weight solution in toluene/hexane over vinyl primer-coated plywood.

Exles 8 and 9 A solution of cellulose diacetate, 2.4 DS acetate at 10% w/w, in ethyl lactate (Example 8) or diacetone alcohol (Example 9) was overcoated by brush onto an epoxy primer coated polycarbonate test panel.

Example- 10

CAP (2.2 DS propionate, 0.23 DS acetate) was applied as a 30% by weight solution in xylene/methoxypropanol 50:50 by volume by brush over tar-free vinyl-coated plywood.

Examiples 11 to 13 Cellulose diacetate, was applied as a 10% by weight solution in acetone containing 10% by weight catalyst based 20 on cellulose diacetate by brush onto epoxy primer coated polycarbonate. The catalysts were tin (II) chloride (Example 11), zinc acetate (Example 12) and magnesium acetate (Example 13).

Ex&mples 14 to 16 CAP (2.2 DS propionate, 0.23 DS acetate)as a 10'-6' by weight solution in methyl isobutyl ketone containing 10% by 11 - weight catalyst based on CAP was applied by brush onto epoxy primer- coated polycarbonate. The catalysts used were tin (II) chloride (Example 14), zinc acetate (Example 15) and magnesium acetate (Example 16).

1

Claims

1. A process for protecting an underwater surface against fouling by aquatic organisms, said process comprising application of a top coat paint over an undercoat on the underwater surface, characterised in that the top coat paint contains as film-forming binder a solution of a polysaccharide ester in a solvent which is a non-solvent for the undercoat under the conditions of application.

2. A process according to claim 1, characterised in that the polysaccharide ester is at least partially a cellulose ester of a carboxylic acid having at least 3 carbon atoms.

3. A process according to claim 2, characterised in that the cellulose ester has a degree of substitution of 1.2 to 3 carboxylate ester groups per anhydroglucose unit, at least half of which are ester groups of a carboxylic acid having at least 3 carbon atoms.

4. A process according to claim 3, characterised in that the cellulose ester has a degree of substitution of at least 2.4 carboxylate ester groups per anhydroglucose unit.

5. A process according to any of claims 2 to 4, characterised in that the cellulose ester top coat is applied from solution in a ketone containing at least 6 carbon atoms, an ester containing at least 4 carbon atoms or an alcohol or ether-alcohol containing at least 4 carbon atoms, or a mixture of said ketone, ester, alcohol or ether-alcohol with an aromatic hydrocarbon.

6. A process according to claim 1, characterised in that the polysaccharide ester is cellulose acetate and the solvent is a hydroxyketone, a diketone or an ester of a hydroxycarboxylic acid.

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7. A process according to any of claims 2 to 6, characterised in that the cellulose ester has a molecular weight in the range 10000 to 50000.

8. A process according to any of claims 1 to 7, characterised in that the top coat paint is applied as a clear coat.

9. A process according to any of claims 1 to 7, characterised in that the top coat paint contains a pigment and comprises a biocide for aquatic organisms.

10. A process according to any of claims 1 to 9, characterised in that the top coat paint contains a substantially non- volatile plasticiser for the polysaccharide ester.

11. A coating composition for protecting an underwater surface against fouling by aquatic organisms, characterised in that the coating composition contains a polysaccharide ester as film-forming binder and also contains a catalyst which accelerates dissolution of the polysaccharide ester when a film of the polysaccharide ester is submerged in water.

12. A coating composition according to claim 11, characterised in that the polysaccharide ester is cellulose acetate.

13. A coating composition according to claim 11, characterised in that the polysaccharide ester is a cellulose ester having a degree of substitution of 1.5 to 3 carboxylate ester groups per anhydroglucose unit, at least half of whichare ester groups of a carboxylic acid having at least 3 carbon atoms.

14. A coating composition according to any of claims 11 to 13, characterised in that the catalyst is a salt of a divalent metal.

14

15. A coating composition according to claim 14, characterised in that the catalyst is magnesium acetate or zinc acetate.

16. A coating composition according to any of claims 11 to 13, characterised in that the catalyst is an oxide of a Group IVB or VB metal.

17. A coating composition according to any of claims 11 to 13, characterised in that the catalyst is a dialkyl hydrogen phosphate.

18. A process according to any of claims i to lo, characterised in that the top coat paint is a coating composition according to any of claims 11 to 17.