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Giving Marine Life The Slip

The ultimate marine paint continues to elude scientists and manufacturers

by Alex Scott
January 19, 2015 | A version of this story appeared in Volume 93, Issue 3

Chemical makers hunt for the ultimate low-friction antifouling marine paint.
Credit: Producer Alex Scott/Editor Linda Wang

Paint companies, and the shippers they serve, have been hunting for the perfect antifouling compound—an ingredient to ward off barnacles, mussels, and other sea life from oceangoing vessels—ever since tributyl tin, a formidable biocide, was banned in 2008 on environmental grounds.

These days, biocidal antifouling compounds often contain elements such as copper and zinc, but manufacturers would still like to find an even more effective and environmentally benign product.

A new approach is emerging. After the International Maritime Organization’s introduction of mandatory greenhouse gas emission reductions for large ships in 2013, leading marine paint firms are no longer using only biocides to keep ships free of organisms and moving easily through the water. Paints that create low-friction surfaces are all the rage.

A coating that cuts surface friction will not only prevent most organisms from attaching to a ship but will also reduce its greenhouse gas emissions and potentially save ship owners thousands of dollars in fuel costs. But a problem that producers of low-friction coatings have yet to solve is how to prevent a buildup of bacterial slime when a ship is stationary or moving slowly.

Among those at the head of the low-friction coatings fleet is AkzoNobel. The Dutch firm claims that a ship coated with its Intersleek 900 fluoropolymer-based paint generates so little friction that it can cut 9% of greenhouse gas emissions from fuel compared with a ship coated with biocidal antifouling paint.

The slippery surface of Intersleek 900 means that once a ship is moving, marine organisms that might otherwise attach are washed off. Other low-friction options include PPG Industries’ silicone-based SigmaGlide 1290 and EnviroMarine from the English firm Brunel Marine Coating Systems.

Shippers switching from biocidal coatings to low-friction alternatives now have the added incentive of gaining carbon credits. For example, AkzoNobel expects two ship owners with a total of 17 vessels to net about $500,000 in carbon credits as a result of switching to Intersleek 900.

But slime can form even on the lowest-friction coatings, hampering their performance over time. Slime can markedly increase a ship’s drag after a year or so, when it can even form a crust, according to Mehmet Atlar, professor of ship hydrodynamics at Newcastle University, in England. “There is a lot of effort being put into finding a solution,” Atlar says, but so far it has proven elusive.

Because of this shortcoming of low-friction coatings, biocidal coatings continue to be favored for ships that spend little time moving quickly over the open ocean.

Credit: Brunel Marine Coatings
Low-friction coatings can reduce ships’ fuel consumption, but their Achilles’ heel is bacterial slime.
Ship with low friction coating.
Credit: Brunel Marine Coatings
Low-friction coatings can reduce ships’ fuel consumption, but their Achilles’ heel is bacterial slime.

“We don’t see low-friction coatings completely replacing biocide-based antifouling either in the short or in the long term,” says Rick Strittmatter, global R&D director for Dow Microbial Control. Dow Chemical’s flagship product is its Sea-Nine 211N broad-spectrum biocide. It features the active ingredient 4,5-dichloro-2-octyl-2H-isothiazole-3-one.

Strittmatter anticipates future use of SeaNine 211N as part of the new, low-friction coatings that are gaining traction in the market. However, not everyone shares his view that biocides and low-friction coatings can be combined.

“It isn’t a logical option” to combine low-friction coatings with biocidal coatings since the latter are designed to shed layers to expose more biocide, says Richard N. Hussey, owner of Brunel. As they shed, biocidal coatings actually lead to rougher surfaces, he adds.

Progress is under way, though, to make low-friction paints even slipperier. PPG has a series of promising developments in the pipeline with patents pending, says Sijmen Visser, global marketing manager for the firm’s marine coatings unit. Meanwhile, at a “slime farm” lab in Newcastle, England, AkzoNobel is testing a range of novel coatings that reduce drag and curb bacterial buildup.

One option the Dutch company has been evaluating is to combine coatings with chemicals that interfere with bacterial communication and prevent slime buildup, says David Williams, R&D director for AkzoNobel Marine Coatings.

But it might just take a greater scientific understanding of the physical conditions at play before the perfect solution is found. To generate this knowledge, the European Union is funding a research project named SeaFront that involves 19 academic and industrial organizations, including AkzoNobel, Solvay, and Atlar’s team at Newcastle University.

SeaFront’s aim is to better understand the coating-biofouling interaction and its impact on hydrodynamic drag by studying surface chemistry, surface structure, biocides, and biobased fouling control systems.

What is already clear is that “the winning technology of the future should be nontoxic, solvent-free, and nonconsumable,” PPG’s Visser says. Shippers are no doubt hoping that SeaFront can fold these attributes into a coating that solves their slippery problem once and for all.



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