IMPROVED ANTIFOULING PAINT COMPOSITIONS

Abstract: Improved anti-fouling paint compositions are disclosed, comprising a film-forming polymer and at least one sparingly soluble metalliferous pigment, said polymer being prepared by polymerization of 20-45 wt % of monomer units A of at least one triorganotin salt of an olefinically unsaturated carboxylic acid, 5-35 wt % of comonomer units B consisting of vinylpyrrolidone, vinylpiperidine and/or vinylcaprolactam, the total A+B being of 40-70 wt %, and the balance being monomer units C of C₁-C₄ alkyl methacrylate and/or styrene and optionally up to 10 wt % of another olefinically unsaturated comonomer. Also disclosed is a process for applying a top-coat paint on a ship hull, said paint having a composition as disclosed hereabove.

Description: This invention relates to marine anti-fouling (AF) paints generally used as top-coat paints for ships' hull, particularly to marine anti-fouling paints which erode at a predetermined rate, and more particularly to an improved polymeric binder for a marine anti-fouling paint which hydrolyzes at a predetermined rate in the presence of sea water. The performance of a ship falls off either when marine organisms grow on the sub-marine parts of her hull, or when the topcoat paint either is applied irregularly or becomes rough during service. The extent of marine fouling is controlled by applying to the hull a top coat paint comprising anti-fouling agents. The anti-fouling agents are biocides which are freed from the paint surface at a rate such that their concentration is lethal to marine organisms near the surface. Anti-fouling paint compositions in which organometallic poisons are chemically bound in the paint vehicle have been known for a long time. British patent 1,124,297 discloses a considerable increase in the anti-fouling efficacy of such paints by using pigments which have an appreciable water-solubility in large quantity by comparison with the quantity of paint vehicle, and particularly if the pigments have a toxicity to marine organisms in their own right. However, the surface of the coating becomes progressively rougher due to the dissolution of part of the composition. British patent 1,457,590 discloses a film-forming copolymer containing 50-80 wt % triorganotin monomer, which gradually dissolves so that (i) a fresh anti-fouling paint surface is continuously revealed, and (ii) moving seawater smooths the surface of the coating. European patent application 51,930 discloses certain polymers of acrylate esters which can be used as the basis of erodible marine paints. The marine paints disclosed comprise a film-forming acrylic polymer and a sparingly soluble metalliferous pigment. The acrylic polymer comprises - 20-100 wt % of monomer units (A) selected from methyl acrylate and ethyl acrylate, - 0- 45 wt % of monomer units (B) of a triorganotin salt of an olefinically unsaturated carboxylic acid, - 0- 80 wt % of monomer units (C) selected from C₁-C₄ alkyl methacrylate and styrene, monomer units (A), (B) and (C) forming at least 80 wt % and the balance, if any, being monomer units of at least one other olefinically unsaturated comonomer. However, marine paints according to EP-51,930 have the disadvantage that they show considerable blistering and flaking. It has also been said that, due to the low hydrolysis rate of this copolymer, the self-polishing effect is not fully satisfactory. A recent Belgian patent, n° 901,426, has again disclosed marine paints comprising a film-forming polymer made from : - up to 60 wt % of triorganotin acrylate or methacrylate, - 1-80 wt % of C₁-C₂ alkoxy C₁-C₂ alkyl(meth)acrylate, and - 20-85 wt % of C₁-C₄ alkyl methacrylate. However, the coatings obtained are relatively soft and therefore are more sensitive to mechanical damages. Moreover, some of the hereabove compositions containing TBTMA and high amounts of alkyl methacrylate do not show a sufficient hydrolysis rate, thus leading to the drawbacks well known in the art, particularly about the anti-fouling properties. Many countries are introducing new stringent water quality programmes, some of which call for a phased reduction in the triorganotin content of copolymer paints. The lowest efficient limit has been determined by T.A. Banfield et al. (J.Oil Col.Chem.Assoc. 63 , 93, 1980) as being of 1 microgram tributyltin oxide/cm².day. The lowest theoretical triorganotin monomer content in the binder may therefore be evaluated at about 10 wt % for an erosion rate of 5 micrometer/month. There is therefore a need in the art for improved erodible anti-fouling paint compositions comprising a film-forming polymer having a reduced content of organotin monomer, while retaining the good anti-fouling and self-polishing properties of the anti-fouling paints of the prior art which contain 50-80 wt % of organotin monomer. An object of the present invention is to provide improved erodible anti-fouling paint compositions for use as top coat paints for ships′ hull. Another object of the invention is to provide an improved polymeric binder for a marine anti-fouling paint which hydrolyzes at a predetermined rate in the presence of seawater. A further object is to provide an improved polymeric binder for an erodible marine anti-fouling paint which has a reduced content of organotin monomer. Accordingly, the present invention provides a marine paint comprising a film-forming polymer, and at least one sparingly soluble metalliferous pigment, characterized in that the polymer is prepared by polymerization of : - from 20 to 45 wt % of monomer units A of at least one triorganotin salt of an olefinically unsaturated carboxylic acid, - from 5 to 35 wt % of at least one comonomer B selected from the group comprising VP (vinylpyrrolidone), VPi (vinylpiperidone) and VC (vinylcaprolactam), the total of A + B being of from 40 to 70 wt % of the polymer composition, and the balance of the polymer being monomer units C of at least one C₁-C₄ alkyl methacrylate and/orstyrene. The Applicant has now unexpectedly found that by using, in an erodible marine paint comprising a film-forming polymer and at least one sparingly soluble metalliferous pigment, a polymer prepared by polymerization of : - from 20 to 45 wt % of monomer units A of at least one triorganotin salt of an olefinically unsaturated carboxylic acid, - from 5 to 35 wt % of at least one comonomer B selected from the group comprising VP (vinylpyrrolidone), VPi (vinylpiperidone) and VC (vinylcaprolactam), the total of A + B being of from 40 to 70 wt % of the polymer composition, and the balance of the polymer being monomer units C of at least one C₁-C₄ alkyl methacrylate and/or styrene. The organotin monomer content was reduced while retaining the good properties of the anti-fouling paints of the prior art which contain 50-80 wt % of organotin monomer. This is really unexpected since the prior art does not lead to low organotin content paint compositions as hereabove described, which give coatings which do not show blistering or flaking while having good anti-fouling and self-polishing properties. The monomer units A are preferably of the formula R₃SnOOCCR′=CHR′′ where each R is an alkyl radical containing 2 to 8 carbon atoms or an aryl or aralkyl radical, R is H or CH₃ and R′′ is H or -COOSnR₃. The groups R can be different but are preferably the same, for example R₃Sn can be tributyl tin, tripropyl tin, tribenzyl tin, triphenyl tin, tritolyl tin or trihexyl tin. The carboxylate group­ -OOCCR′=CHR′′ is preferably acrylate, methacrylate or maleate. The polymer comprises from 20 to 45 wt % and preferably from 30 to 45 wt % of monomer units A. As monomer B, there may be used VP and/or VPi and/or VC, but preferably VP. The polymer comprises from 5 to 35 wt % of monomer B, preferably from 10 to 25 wt %. Amounts higher than 35 wt % give excessively hydrophilic resins which become swollen and lose their mechanical properties; also, the erosion rate observed is excessively high, which is economically unacceptable. The total of A + B amounts to from 40 to 70 wt % of the polymer composition, preferably from 50 to 65 wt %. This range is determined so as to give an optimal range of polishing effect. As monomer C, there may be used one or several C₁-C₄ alkyl methacrylates and/or styrene, but preferably one or several of said methacrylates, including ethylmethacrylate (EMA) and methylmethacrylate (MMA), the most preferred being MMA. The monomer units C can be replaced by less than 10 wt % with respect to the monomer composition of one or several olefinically unsaturated comonomers other than those hereabove described as suitable for A, B or C. Higher amounts would give coatings which show flaking or blistering. As examples of suitable comonomers, it may be cited EA (ethyl acrylate), MA (methyl acrylate), PA (propyl acrylate), BA (butylacrylate), HA (hexyl acrylate), EHA (2-ethylhexyl acrylate), AN (acrylonitrile), MAN (methacrylonitrile), acrylamides such as DAAM (diacetone acrylamide) or NMA (N-­ methylolacrylamide), vinyl monomers such as VPy (vinyl pyridine), vinyl chloride, vinyl acetate, vinyl propionate or vinyl butyrate, methylvinylether, cyclohexene, vinylidene chloride, and otheranalogs. The polymer can be prepared by addition polymerisation of the appropriate monomers in the appropriate proportions at polymerisation conditions using a free radical catalyst such as benzoyl peroxide, t-butyl peroxy 2-ethyl hexanoate (TBPEH), t-butyl peroxybenzoate (TBP), or azobisisobutyronitrile in an organic solvent such as xylene, toluene, butyl acetate, n-butanol, 2-ethoxyethanol, cyclohexanone, 2-methoxyethanol, 2-butoxyethanol and/or 2-­ ethoxyethyl acetate. Polymerisation is preferably at a temperature in the range 70°-140°C although higher temperatures may be used providing that the solvent and the catalyst are adapted thereto. Within this range the use of higher temperatures produces polymers of lower molecular weight. Polymerisation may be carried out by heating all the polymer ingredients in the solvent or preferably by gradually adding the monomers and catalyst to the heated solvent. The latter procedure produces polymers of lower molecular weight. A chain transfer agent such as a mercaptan can also be used to produce polymers of lower molecular weight. For a given proportion of monomers the paint tends to dissolve in and become smoothed by relatively moving sea water more rapidly if the polymer has a lower molecular weight, and such polymers may be preferred when using a high proportion of monomer units C, particularly if monomer units C comprise styrene. However, one skilled in the art knows that the molecular weight of the polymer must be sufficient to have film-forming properties. Mixtures of polymers of different composition, or of higher and lower molecular weight polymers, can be used. Alternatively the paint can be an aqueous emulsion paint based on a polymer prepared by emulsion polymerisation. The metalliferous pigment sparingly soluble in seawater is exemplified by cuprous thiocyanate, cuprous oxide, zinc oxide, cupric acetate meta-arsenate, zinc chromate, zinc dimethyl dithiocarbamate, zinc ethylene bis(dithiocarbamate) and zinc diethyl dithiocarbamate. The paint preferably includes at least one metalliferous pigment selected from zinc oxide, cuprous oxide and cuprous thiocyanate and pigments whose seawater solubility is similar to theirs. These pigments have a seawater solubility such that the pigment particles do not survive at the paint surface. The pigment has the effect of inducing the overall smoothing which the relatively-moving seawater exerts on the paint film, minimising localised erosion and preferentially removing excrescences formed during the application of the paint. Mixtures of sparingly soluble pigments can be used, for example zinc oxide which is most effective at inducing the gradual dissolution of the paint, can be mixed with cuprous oxide, cuprous thiocyanate, zinc dimethyl or diethyl dithiocarbamate, or zinc ethylene bis-­ (dithiocarbamate) which are more effective marine biocides. Such mixtures preferably comprise up to 75 percent by volume of zinc oxide and 25-100 percent by volume of cuprous oxide or thiocyanate. Indeed, it is known that synergy and complementary effects are observed when using these pigments. Zinc dimethyl dithiocarbamate, zinc ethylene bis(dithiocarbamate) and zinc diethyl dithiocarbamate are somewhat less soluble than cuprous oxide or thiocyanate. If used, they preferably form up to 75 percent by volume of the pigment, with at least 25 percent by volume of the pigment being zinc oxide. The paint composition can additionally contain a pigment which is highly insoluble in seawater, such as titanium dioxide or ferric oxide. Such highly insoluble pigments can be used at up to 40 percent by weight of the total pigment component of the paint. The highly insoluble pigment has the effect of retarding the dissolution of the paint. The proportion of pigment to polymer is generally such as to give a pigment volume concentration of at least 25 percent, preferably at least 35 percent, in the dry paint film. The upper limit of pigment concentration is the critical pigment volume concentration. Paints having pigment volume concentrations of about 50 percent, for example, have been found very effective for smoothing in seawater and preventing fouling. The paint can contain other biocides effective as antifouling agents. As examples of such biocides, it may be cited di-thiocarbamate derivatives such as cuprous ethylene bis-di-thiocarbamate or 2-(N,N-dimethyl thiocarbamyl thio)-5-nitro thiazole, substituted isothiazolones particularly halogenated N-substituted isothiazolones, tetramethyl thiuram disulphide. The invention is also illustrated by means of the following examples which are meant to be descriptive and by no means limiting. Example 1 A copolymer has been produced by gradually adding - 37.5 g TBTMA - 42.5 g MMA - 7.5 g EHA - 12.5 g VP - 1.5 g TBPEH to 100 g xylene heated at 85°C while stirring. After about 12 hours heating, no further polymerization has been observed by monitoring residual monomers by gas chromatography. The solution has then been heated to 110°C for about one hour to destroy any residual activity of the catalyst. The product was an approximately 50 wt % solution of copolymer in xylene. 40 g of the 50 wt % solution of copolymer in xylene have been blended with 2 g of antisettling agents and stabilizers, 40 g of pigments and 20 g of additional xylene. The pigment component comprised 54 vol.% ZnO, 27 vol. % zinc ethylene bis(dithiocarbamate), 9 vol. % Cu₂O, 6 vol% TiO₂ and 4 vol. % of additives and stabilizers. The anti-fouling paint thus obtained has been tested for its erosion properties according to the following procedure. A steel disc of 20 cm diameter has first been coated with a 220 µm dry film thickness layer of a commercially available anti-­ corrosive paint based on coal tar and epoxy resin. The anti-­ fouling paint has been applied as a layer having a dry film thickness of 120 µm within 48 hours of the drying of the anti-­ corrosive layer. After drying, the disc has been rotated in a tank filled with continuously renewed natural seawater at a temperature of 25°C. The circumferential speed of the disc was of about 27 knots (50 km/h). The first thickness measurements were made after seven days conditioning, then 270 days thereafter, on 10 different places at 9 cm from the discs center. The average erosion rate was of 6.1 µm/month. The paint thus obtained has also been tested for anti-fouling activity by applying it to a plaque over a vinyl resins/tar anti-corrosive paint, mounting the plaque on a panel and immersing the panel from a raft in a river estuary off Southern Netherlands during 12 months. Each panel also included plaques coated with a non-toxic control (which became heavily fouled with seaweed and some animal life within 4 weeks). No weed or animal fouling has been observed on the panels coated with the paint of the invention, and the algal slime was practically non-existent. Examples 2 to 12 and additional examples 1 to 5. The paints of these examples have been produced and tested for erosion rate according to the procedure of example 1. The compositions and the results of the erosion rate test are mentioned in the Table. The compositions used in the additional examples 1 to 5 are outside the scope of the present invention. The paints of examples 2 and 3 have further been tested for their anti-fouling properties according to the procedure described in example 1. No weed nor animal fouling has been observed after twelve months exposure. Additional example 6 The erosion rate of a commercially successful paint has been tested according to the procedure of example 1, with a result as indicated in the Table.

Claims: 1. Marine paint comprising a film-forming polymer, and at least one sparingly soluble metalliferous pigment, characterized in that the polymer is prepared by polymerization of : - from 20 to 45 wt % of monomer units A of at least one triorganotin salt of an olefinically unsaturated carboxylic acid, - from 5 to 35 wt % of at least one comonomer B selected from the group comprising VP (vinylpyrrolidone), VPi (vinylpiperidone) and VC (vinylcaprolactam), the total of A + B being of from 40 to 70 wt% of the polymer composition, and the balance of the polymer being monomer units C of at least one C₁-C₄ alkyl methacrylate and/or styrene. 2. Marine paint according to claim 1, wherein the proportion of monomer units A is of from 30 to 45 wt %. 3. Marine paint according to claim 1, wherein the proportion of monomer units B is of from 10 to 25 wt %. 4. Marine paint according to claim 1, wherein the total proportion of monomer units A and B is of from 50 to 65 wt %. 5. Marine paint according to any one of claims 1 to 4, wherein the monomer unit A is TBTMA, the monomer unit B is VP and the monomer unit C is MMA. 6. Marine paint according to any one of claims 1 to 5, wherein less than 10 wt % on the monomer composition of monomer units C are replaced by one or several olefinically unsaturated comonomers selected from the group comprising EA, MA, PA, EHA, AN, MAN, DAAM, NMA, VPy and vinyl propionate. 7. Coating of a ship hull, whenever obtained by using a marine paint according to any one of the preceeding claims. Claims for the following Contracting State : ES. 1. Process for applying a top-coat paint on a ship hull, said paint comprising a film-forming polymer and at least one sparingly soluble metalliferous pigment, characterized in that the film-forming polymer is prepared by polymerization of : - from 20 to 45 wt % of monomer units A of at least one triorganotin salt of an olefinically unsaturated carboxylic acid, - from 5 to 35 wt % of at least one comonomer B selected from the group comprising VP (vinylpyrrolidone), VPi (vinylpiperidone) and VC (vinylcaprolactam), the total of A + B being of from 40 to 70 wt% of the polymer composition, and the balance of the polymer being monomer units C of at least one C₁-C₄ alkyl methacrylate and/or styrene. 2. Process according to claim 1, wherein the proportion of monomer units A is of from 30 to 45 wt %. 3. Process according to claim 1, wherein the proportion of monomer units B is of from 10 to 25 wt %. 4. Process according to claim 1, wherein the total proportion of monomer units A and B is of from 50 to 65 wt %. 5. Process according to any one of claims 1 to 4, wherein the monomer unit A is TBTMA, the monomer unit B is VP and the monomer unit C is MMA. 6. Process according to any one of claims 1 to 5, wherein less than 10 wt % on the monomer composition of monomer units C are replaced by one or several olefinically unsaturated comonomers selected from the group comprising EA, MA, PA, EHA, AN, MAN, DAAM, NMA, VPy and vinyl propionate. 7. Coating of a ship hull, whenever obtained by a process according to any one of the preceeding claims.

Inventors: BRAEKEN, JOZEF, DEKERK, JEAN-PAUL, VAN DER POEL, HENDRIK, VERBORGT, JOZEF
Applicants: FINA RESEARCH S.A.
Application year: 1986
Main classification: C09D5
IPC codes: C08F26, C08F230, C08F226, C08F30, C08F220, C09D5