When comparing different types of antifouling paints against fouling on leisure boats, the results were the opposite of what many would expect. Of the paints tested, the biocide-free silicone paint worked best, and the paint marketed as environmentally friendly turned out to be extremely toxic.
Contaminating biocides
An untreated boat hull can be covered in a thick layer of algae, barnacles and other marine organisms in just a few weeks. This makes the boat both heavier and slower in the water – which leads to higher fuel consumption, among other things. Therefore, boat owners regularly paint the hull with so-called “antifouling paints” that slowly release toxic substances – biocides – to prevent the organisms from growing on the hull.
The paints that dominate the market contain the biocide cuprous oxide and are effective as long as they release copper. However, it has two major disadvantages – it affects other organisms in the environment, and it contaminates the water and bottom sediments.
‘The goal of our study was to see how the environmental impact of antifouling paints can be reduced, while boat owners get the effect they want,’ says Maria Lagerström, researcher at the Division of Maritime Environmental Science at Chalmers and lead author of the new study.
Seven different paints in different marine environments
Together with colleagues from Swedish, Danish and French universities, Lagerström investigated seven different antifouling paints:
The last paint is not sold in Sweden today, but in several other EU countries, including Denmark and France, where it is marketed as safe for the environment.
Testing with panels
To test the effectiveness, the different paints were applied to panels that were immersed in the water for up to six months in three different coastal waters:
The researchers measured both the surface coverage of the fouling on the different paints, and how much biocides were released into the surrounding water.
Although the coastal waters in the different countries differ, all the results pointed in the same direction: the silicone-based biocide-free paint was the most effective, closely followed by the biocide-containing tralopyril paint. For the copper coatings, the copper content in the paint was not a deciding factor, meaning that each of the five copper paints had more or less the same effect against fouling, despite a wide range in their copper content.
‘Eco-friendly’ paint most toxic
The researchers also performed two different tests to see how much the paints impact the marine environment. The first was an environmental risk assessment where models were used to assess the levels of biocides that the paints can give rise to in the marine environment, based on how much was released into the water during the test period. The silicone paint was not included here, as it does not contain biocides.
According to the test, none of the biocidal paints would meet today’s environmental risk criteria, even if those with a low copper content fell below limit values in French waters. But one paint clearly stood out – the tralopyril paint, which is marketed as environmentally friendly.
‘I’ve done this kind of environmental risk model before, but I’ve never seen such high values.
The amount of biocide released was several thousand times higher than the acceptable levels, so I’m surprised that this paint is promoted as environmentally safe,’ says Lagerström.
The second test included all paints, where four different marine species were exposed to leachate solutions from the painted panels in the lab to see how they were affected. In this case, too, it was the paint with tralopyril that fared the worst, followed by the paint with the highest copper content.
Copper-free more effective
‘If we sum up the study, we can conclude that the copper-free alternatives were actually more effective than those containing copper – but with very different effects on the environment. The test winner is the silicone paint that has the best sustainability profile because it performed well against fouling while having low toxicity. It requires more prep work and a special underlying coating to get the paint to adhere to the hull, but in our comparison it looks extremely promising and is worth trying.’
Lagerström adds: ‘Using paints with the highest copper content would have an unnecessary environmental impact now that we have shown that it works with lower levels. If we phase them out, it would make a huge difference to the environment in our European coastal waters. And I really can’t see any reason why highly toxic products like the tralopyril paint should be allowed on the market in any European country at all.’
