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“Green” hull paints found to contain toxins banned for causing sex change

Premium ship hull coatings from the world’s two largest manufacturers, currently promoted as ‘green’ alternatives, have been found to contain toxic compounds banned as anti-fouling agents by the UN’s International Maritime Organisation (IMO).

Independent testing of one coating, marketed as “biocide-free”, has revealed significant amounts of toxic tin compounds, despite all use of tin as an anti-fouling agent being strictly banned since 2008. The IMO ban on tin-based anti-fouling came after the discovery that it was causing ‘chemical castration’ of key marine species, including causing female snails to grow a penis.

Lab results find tin

Lab results seen by Wikigreen have confirmed suggestions that the leading silicone “foul-release” hull coating, Intersleek 1100SR, contains significant amounts of organotin. A sample of Intersleek 1100sr, manufactured by International Paints (an AkzoNobel company), was recently analysed by spectometer at a respected European lab. Results found 12730 mg of tin per kg of wet paint.

After evaporation of solvents, the tin content of the dried coating would be higher, leaving in the region of a third of a kilogram of tin from a 20-litre can of paint. Since an average size cruise vessel might need 8500 liters of hull paint, 132 kg of tin compounds could be contained in the coating – a volume of serious concern to marine biologists.

International Paints have responded that the quantity added is 1,000 mg per kg, (0.1%), below the 2,500 mg per kg limit allowable under the IMO Anti Fouling Convention, and that the use of organotin catalysts in foul release coatings is allowable within limits stated Convention. They also state that the catalyst is bound in the polymer matrix of the final product. Marine biologists however are concerned that, even falling within the legal limits for a catalyst, the tin content is high enough to act as a biocide.

Unwelcome return

It has been generally assumed that tin contamination of the oceans by shipping had ended after the IMO ban came into effect. It now appears that AkzoNobel, and possibly other anti-fouling manufacturers, may be continuing to use it in amounts large enough to have toxic effects on marine life whilst arguably staying within the letter of the law.

Organotin creates imposex in sea snails and other species: female sea snails (marine gastropod molluscs) develop male sex organs such as a penis and a vas deferens. (1)

An apparent loophole in the wording of the 2008 IMO ban on tin products can be interpreted as failing to completely ban all traces of tin in anti-fouling coatings. The ban explicitly includes “Organotin compounds which act as biocides in antifouling systems”.

While Akzo Nobel  acknowledge the presence of tin in their premium ‘foul-release’ coating, its presence is said to be as a left-over catalyst from the production process rather than intentionally as a biocide.

Product literature from International Paints states: “This product does not contain organotin compounds acting as biocides and as such is in compliance with the International Convention on the Control of Harmful Anti-fouling Systems on ships as adopted by IMO in October 2001”.

Tacit admission

Safety Data Sheets for the product issued by AkzoNobel reveal the presence of ‘dioctyltin dilaurate’ at between 5 and 10% by weight in one of the coatings three constituent mixing packs. Dioctyltin compounds, along with other organotin compounds, are considered to be “highly toxic to aquatic organisms” in the words of manufacturers such as Dow Chemicals. The volume of tin admitted to be present in Intersleek 1100SR is broadly in line with the recent independent lab analysis.

Rival manufacturer Hempel also produce silicone-based hull coatings with similar chemistry, and admit in Safety Data Sheets to the presence of 3 – 5% of ‘dibutyltin dilaurate’ in the curing agent of their premium product HEMPAGUARD X7 89900. Although not included in the recent independent test, this similar quantity would be expected to result in a similar amount of tin in the finished paint. Hempel also rely on a disclaimer concerning organotin virtually identical to that of AkzoNobel. While tin is often used as a catalyst in the production of silicone products there are other common alternatives.

Dogwhelks, a key indicator species which revealed toxic marine effects of tin anti-fouling. Photo: By Martin Talbot – originally posted to Flickr as Winkles, CC BY 2.0,

Dogwhelks, a key indicator species which revealed toxic marine effects of tin anti-fouling. (2)

Biological hazard

Regardless of how the tin got there, marine biologists say its presence in these volumes is far above the reasonable concentration that would be expected for a catalyst. More importantly, they say it is more than enough to have a biocidal effect, and to seriously add to the toxic legacy of tin pollution which still affects key species of marine life. Residues of tin have left some harbour sediments so toxic that port authorities dare not dredge them for fear of bringing the tin back into circulation in the water.

Concern has been raised by several researchers on the use of organotins as catalysts in the production of silicone hull coatings, suggesting that amounts greater than 2500mg of tin per kilogram of dry paint would be a reasonable limit for purely catalytic purposes. The recent lab tests suggest more than 100 times that concentration. Researchers also point out that water abrasion as well as ice and debris impact leads to shedding of the coating directly into the water.(3)

Additional research into the mechanism which lessens the ability of barnacles to adhere to silicone coatings suggests that it is at least partly due to chemical activity, altering the curing of glues and/or enzyme activity.(4) These conclusions lend further weight to the suggestion that the presence of tin may be biocidal.

The oils in silicone-based hull coatings are also a cause for concern. Other research has shown that they inhibit the development of sea urchins, through trapping and suffocation. These oils themselves might be considered biocidal. (5)

Hypocrisy charges

Any manufacturer found to be violating the ban could expect serious consequences, as could shipowners who have been using the coatings. The test results also open International/Akzo Nobel and possibly other manufacturers to charges of corporate hypocrisy: the Intersleek coatings are marketed as a “green” and “biocide-free” alternative to the more common copper-based anti-fouling coatings, which are themselves under attack on environmental grounds.

Tin-based anti-fouling was banned in 2008 after it was discovered to be causing reproductive damage in key species of marine flora and fauna.

Wikipedia notes:

TBT compounds are banned and are included in the Rotterdam Convention and have been banned by the International Convention on the Control of Harmful Anti-fouling Systems on Ships of the International Maritime Organization.

Bans on TBT on boats less than 25 metres long first started in the 1980s. In 1990, the Marine Environment Protection Committee adopted Resolution MEPC 46(30), which recommended that the Government eliminate the use of TBT-containing antifouling paints on smaller vessels. This resolution was intended to be a temporary restriction until the International Maritime Organization could implement a complete ban of TBT anti-fouling agents for ships. Several countries followed with a ban of use, and in 1997 Japan banned the production of TBT-based anti-fouling paints.

The use of organotin compounds acting as biocide in anti-fouling paint was completely banned in 2008 by the International Convention on the Control of Harmful Anti-fouling Systems on Ships of the International Maritime Organization. It states that ships cannot bear organotin compounds on their hulls or external parts or surfaces unless there is a coating that forms a barrier so that organotin compounds cannot leach out. This measure helps reduce exposure by allowing recovery to occur. Despite the ban, TBT will most likely be present in the water column and sediment for up to twenty years because of its long half-life


Unpublished research into the presence of tin in ‘foul-release’ coatings is summarised in our earlier article here.


  1. Creative Commons photo:
  2. Photo: By Martin Talbot – originally posted to Flickr as Winkles, CC BY 2.0,
  3. Published research by Nehring, 2001; Watermann et al., 2005; Chambers et al., 2006; Rittschof et al., 2011
  4. Published research: Compounds from Silicones Alter Enzyme Activity in Curing Barnacle Glue and Model Enzymes. Daniel Rittschof, Beatriz Orihuela, Tilmann Harder, Shane Stafslien, Bret Chisholm, Gary H. Dickinson
  5. Wikigreen report here

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