The fouling of ships’ hulls, whether by barnacles and seaweed or by slime-creating bacteria, is a major problem for shipping worldwide, and particularly for navies.
It has been estimated that fouling of hulls can create such turbulence as a ship moves through the water that fuel consumption is increased by as much as 30 percent.
Traditionally major users of ships, like the US Navy, have attempted to resist fouling by painting hulls with paints containing copper or triorganotin, a tin-based compound. But these paints are highly toxic and can leach into the water, killing marine life. That’s why their use increasingly is being prohibited.
But help is at hand: A research group at Cornell University, Ithaca, N.Y., led by Christopher Ober, has developed two types of non-toxic paint, one hydrophilic and one hydrophobic, that effectively prevent fouling, whether by bacteria or barnacles.
The paints act not only by minimising adhesion by organisms but also by enabling hulls to become self-cleaning: As a ship moves through the water at 10 to 15 knots, the turbulence created removes the clinging barnacle or seaweed.
A report on a solution to marine fouling is being presented in three papers by Ober, the Francis Norwood Bard Professor of Materials Engineering, and by postdoctoral colleagues Luisa Andruzzi and Jeffrey Youngblood at the 225th national meeting of the American Chemical Society in New Orleans.
Ober has been investigating the problem of marine fouling for the past decade for the US Office of Naval Research (ONR). ‘Our ability to engineer surfaces has improved dramatically over the past 10 years, and we have learned that not only do you have to control surface energy and surface chemistry, but also mechanical properties,’ says Ober.
His earliest research was with hydrophobic materials, based on the US Navy’s experience with water-repellent silicone rubber. Two years ago, taking a cue from materials used in surgical implants, he began experimenting with hydrophilic materials. Although these materials attract water, in doing so a very thin boundary of water is formed that marine organisms can’t penetrate. ‘This layer makes it seem to marine organisms that there’s no point in settling there,’ notes Ober.
Both the hydrophilic and hydrophobic materials, which can be spray-painted or applied as a film, use the same commercial block copolymer rubber, which provides mechanical properties. The other part of the coating is a surface-active liquid crystalline structure, designed by the Ober group, which determines whether the paint will be hydrophilic or hydrophobic.
Ober notes that both surfaces have their advantages. ‘What prevents bacteria from adhering might be different from what prevents a full-grown barnacle,’ he says. Unlike barnacles or kelp, bacteria are not dislodged by large flows of water going by. That’s where a hydrophilic surface is appropriate, because it denies bacteria a compatible surface to grow on. ‘The beautiful thing about our system is that we can match mechanical properties exactly, then look for detailed differences in chemical and surface structure,’ Ober says.
Both surfaces are currently being tested by the ONR and by the Ober team’s collaborators, Jim Callow, John Finlay and Maureen Callow at the University of Birmingham, England.
Further information can be found on the web at: <A HREF=’http://www.ccmr.cornell.edu/~cober/’>Ober Research Group</A>