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Antifouling Coatings

By 1870 there were more than 300 antifouling paint registered. Most of the formulations used biocides to kill organisms through a leaching process. In about 1910 pine gum rosin was reported to be used to make a slowly water soluble antifouling. (reference)

R. Mallet a 19th century paint expert noted: “The necessary balance between adhesion and slow diffusion or washing away through the water of the poisonous soap is too delicate for practice. Either the soap adheres firmly and does not wash away enough to keep off fouling, or it washes away so fast as soon to be gone.”

Various heavy metals have mainly been added to give the marine paint antifouling and anticorrosive properties. Marine paints have also been added pigments containing heavy metal. Copper has been the traditional compound used as a biocide in antifouling paints. The concentration of copper in antifouling paints is reported to be 10-30 %, but as much as 50 % has been used. Normally Cu2O is used as the biocide, but also CuSCN and copper metal are in use. (Internet reference 13)

Mercury was added to antifouling paints as a biocide in the past. Both inorganic mercury and organomercury were used. The concentration of mercury in paints is somewhat uncertain but about 5 % could have been added to the paint. Arsenic was also used in the past as a biocide in antifouling paints.

Lead has been added to antifouling paints as a stabilizer, a pigment and a biocide. Both inorganic and organolead compounds are used in marine paints. The concentration of lead in antifouling paints is typically 1-5 %. Lead was previously widely used in anticorrosive paints, but has now to a great extent been replaced by zinc and aluminium. Cadmium and chromium were also in the past added to give the paint anticorrosive properties and colour.

In the late 60's organotin compounds replaced the traditional copper-containing antifouling paints, because of its excellent antifouling properties. Tributyl tin (TBT) is the most used organotin compound but also triphenyl tin (TPT) is used. The amount of organotin compounds in paints is normally about 10-15 %. The use of organotin compounds was banned in most of the world in the 80's for vessels of length less than 25 meters and were replaced by other organic biocides. However, organotin compounds are still widely used on large ocean-going vessels.

Other organic biocides were used as a replacement for organotin. Several pesticides are used as a biocide in antifoulants. Examples of pesticides in use are Diuron and Zineb. DDT was also used as a biocide in antifoulants in the past and it is possible that other persistent chlorinated pesticides have been used in antifoulants.

Later the antifouling paints evolved and used vinyl or chlorinated paint filled with copper and biocides. Some of the biocides used were persistent and accumulated in the water. It was not until 1974 with the evolution of the at that time regarded more environmentally friendly TBT-copolymer systems that we were provided with a real predictable long term anti-fouling performance. Due to the fine and predictable delivery mechanism in this technology the need for biocides was reduced and those used were in general biodegradable. Due to concerns of TBT adhesion to silt, and lasting problems (real or perceived) the TBT-copolymer AF came under scrutiny by environmental groups, and this biocide is now practically banned. In its place has come a reintroduction of the pine rosin systems, under a new name, and some different “new generation” antifouling paints. In brief the present situation in relation to anti fouling paints from a ship owners point of view can be summarized as less predictable, more complicated and more expensive than has been the case since 1974.

It is clear that toxic compounds used previously and today in marine paints are responsible for some of the present marine pollution problems in coastal waters. Antifouling paints are the source of most of the contamination of organotin compounds in harbor basins. Large amounts of copper and to some extent lead and mercury found in the sediments originate from these paints as well. There is also evidence that a part of the PCB contamination in harbor sediments has come from marine paints and will continue to do so due to the small, but not insignificant, amounts of PCB that are still incorporated in the coating of older ships. Paint samples removed from a Norwegian naval vessel in 1996 contained up to 270 mg PCB/kg paint residue on a dry weight basis.

Prevention of further contamination of the marine environment in harbors requires collection of as much as possible of the paint residues from sand blasting, high pressure water cleaning and paint scraping during ship repairs. Removal of contaminated sediment is probably more costly and not near as effective as preventive measures at the source. Even if all the waste is collected at ship repair yards, toxic compounds in antifouling paints will be released continuously from the hull and contaminate harbor sediments. It is therefore necessary to develop antifouling paints containing toxic compounds, which have specific effects on target organisms and are easily degraded in the environment.