Plastic wards off barnacles
Polymer technology could speed ships by stopping critters clinging on.
Barnacles will no longer slow down ships if the latest polymer coatings being developed live up to expectations.
Critters that grow on the hulls of ships — known as fouling — come in two varieties; soft and hard. Soft fouling creatures are slime-growing algae or bacteria. More problematic are hard foulers — barnacles or other sea creatures with a hard shell that cling to the layer of soft slime, or to the ship directly. These lumpy nuisances slow down ships, and cost the US navy up to $1 billion a year. But help is at hand in the form of a polymer coating that the barnacles just can't get a grip on.
Craig Weinman, a graduate student at Cornell University in Ithaca, New York, has developed a polymer that has both water-hating and water-loving components, each of which serve to repel different sorts of creatures. This system has already been shown to prevent soft fouling1; now Weinman has taken it a step further and stopped barnacles from taking root.
The technique involves first coating a surface with a thick layer of a styrene polymer — such as those used in the soles of running shoes — and then slapping on a thin layer, just a few micrometres thick, of the new polymer. This has been modified from a commercially available product, mainly by adding fluorinated groups to affect the surface properties of the plastic.
Weinman tested the coating on a piece of glass, comparing it with another polymer-coated bit of glass that wasn't designed to keep barnacles at bay. He put a droplet of water containing 20-40 barnacle larvae onto these coated glass slides, and then waited 72 hours or until the control was half covered in barnacles, whichever came first. The anti-fouling polymer successfully kept the glass entirely barnacle free.
Slip sliding away
Weinman's approach is passive — it makes the surface as difficult as possible for barnacles to get a hold on, rather than poisoning them. But it isn't clear how, exactly, the chemistry of the surface makes it impossible for the barnacles to hold on. "We don't have a handle on the biology yet," Weinman says.
The main way that ship owners have tried to keep fouling at bay in the past is by coating a hull with the chemical tributyl tin, but this proved so toxic to marine life that it has now been banned. This has left the industry in need of friendlier anti-fouling coatings. Some other paints are based on copper, which also works by poisoning creatures that get too close. A silicon-based polymer system, polydimethylsiloxane (PDMS) has also been developed; in comparison trials, Weinman says his polymer proved the more slippery.
And when Weinman's samples were tested on brine shrimp, they survived happily for two days in water with the polymer.
Weinman's polymer is currently being tested on sample pieces of material in the ocean off the Californian coast. This real-life test will be crucial, says David Schiraldi at Case Western Reserve University in Cleveland, Ohio. "Mother Nature will always try to find a way to foul you," he says.
It's important to make these tests last for at least a year, he adds, because barnacles and other fouling creatures are very season dependent. "You've got to go through a whole year cycle."
Schiraldi is involved in a start-up company looking at another method to control hard fouling creatures — by making the soft foulers grow in such a way that the coating they produce stops the barnacles gripping onto them. But Schiraldi is impressed by Weinman's results. "This is as good as I've seen," he says.
- Krishnan S., et al. Langmuir, 22 . 5075 - 5086 (2006).
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