Antifouling coatings cling to copper

Copper coatings keep ships clear from unwanted sea life but environmental concerns mean some are keen to move away from the metal. Can it be replaced?

Ten years ago, the International Maritime Organisation (IMO) placed a global ban on the use of tributyltin (TBT) in antifouling systems and put an end to what once was among the most important chemicals in the shipping industry. TBT – a group of compounds that consist of between one and four organic components attached to a tin atom via carbon–tin covalent bonds – used to do its job well. It was highly effective at warding off barnacles, shells and other marine creatures, which have the habit of settling onto boat hulls. When sea life smother their hulls, the boats slow down and use more fuel, so people have long hunted for ways to keep tenacious little hitchhikers off their boats.

But leaching out from antifouling paints into the marine environment, TBT turned out to be highly toxic toward non-target organisms. The resulting ban on the substance forced the antifouling industry to make a U-turn.

It became highly reliant on copper, a veteran in protecting boats. As early as the 18th century, hulls were covered in a thin layer of the compound where, in reaction with seawater, it would form copper oxychloride that deters marine creatures from getting a grip. Now, most antifouling products consist of copper metal oxides in combination with other co-biocides, such as zinc pyrithione or the polymer zineb.

But like TBT before it, copper is falling out of favour. Its widespread use in commercial shipping and on leisure boats means worryingly large amounts of copper are leaching from coatings and stoking environmental concerns.

‘Antifouling paints that are meant to end up in the water cannot be part of a sustainable future,’ says Joke Wezenbeek from the Dutch National Institute for Public Health and the Environment (RIVM). ‘Copper and zinc are among the compounds that most frequently exceed the water quality standards in Dutch national legislation and under the EU water framework directive.’

In a study released last year, RIVM warned that antifouling coatings are a significant source of emissions of toxic substances.1 A few months before, Germany’s environment agency (UBA) had found that antifouling paints on recreational boats alone pollute German waters with about 70 tonnes of copper per year.2 Alarmed by what this could do to bacteria, algae and fish in the marine ecosystem, the UBA asked boat owners to hand clean boats of algae and other unwanted growth rather than rely on biocidal paint.

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V C Zainzinger, Antifouling coatings cling to copper, RSC, 2019 - Reproduced by permission of The Royal Society of Chemistry

The full article can be found at chemistryworld.com