Seeking nontoxic coatings to keep ship hulls clean

In March 2017, after 40 years of service, the massive aircraft carrier USS Independence took its final voyage—sailing from the Puget Sound Naval Shipyard in Washington State, around the tip of South America, to a ship-breaking yard in Texas, where it would be dismantled. To reduce the risk that the ship would transport invasive marine species, underwater divers spent 2 months before the trip scraping the surface of the more than 300 m long hull.

The hull of the Independence, like those of all US Navy ships and almost every commercial and recreational vessel around the globe, was coated in antifouling paint to reduce the growth of algae, mussels, barnacles, and other marine life on its surface. That antifouling paint, as most do, contained copper and zinc as its active ingredients. Such coatings help the environment by reducing drag and improving fuel economy, thus reducing greenhouse gas emissions. At the same time, biocidal antifouling paints are, by definition, toxic to marine organisms.

The scraping of the Independence released an estimated 581 m³, or about 73 dump-truck loads, of hull debris contaminated with copper and zinc into Puget Sound, an act for which the navy was successfully sued by the resident Suquamish Tribe and two environmental groups. In trying to prevent ecological damage caused by the spread of invasive species, the navy had polluted Puget Sound with toxic metals.

Researchers working on antifouling coatings are trying to minimize such trade-offs by developing new compounds that serve the needs of commercial and military fleets while mitigating damage to the environment. To be widely adopted, however, these materials will need to be practical and cost effective.

A necessary evil?

Copper was one of the first antifouling surfaces to be widely used. Starting in the mid-18th century, copper sheets were attached to the hulls of wooden ships to prevent barnacles from eating away the wood. By the mid-19th century, iron ships started dominating the seas, but putting copper sheets on iron hulls that sailed through salty seawater set up the perfect conditions for severe galvanic corrosion. The problem was so bad that the British navy considered selling all its iron ships and returning to wooden ones.

Eventually, shipbuilders developed antifouling paints that could be laced with any number of toxic substances. In the 1960s and ’70s, paint containing tributyltin (TBT) became popular because it was cheaper, more effective, and easier to apply than paint containing copper.

But TBT started showing up in every part of the food chain, harming species big and small—sea otters, dolphins, oysters, snails, and more. In 2008, the International Maritime Organization banned the use of TBT in antifouling paints. Other biocides, primarily copper-containing compounds, then reemerged, but they all present their own toxicity challenges to wildlife as well as to the humans who manufacture and apply them.

Timothy Sullivan, head of the Materials and Environmental Science Applications Research Group at University College Cork, has direct experience with some of those concerns. Before becoming an antifouling researcher, he worked for a boatyard as a hull cleaner, and he distinctly remembers the noxious nature of the coatings. “Not only have they got biocides in them, but they’ve also got plenty of solvents and other unhealthy things,” he says.

But the environmental cost of insufficient fouling mitigation is, arguably, even higher. Today, about 90% of global trade happens by sea, producing almost 3% of the world’s human-made carbon dioxide emissions. Fouling on the hull of a massive cargo ship can reduce its fuel economy by as much as 50%, according to BASF. In a study commissioned by the Swedish biotechnology company I-Tech, more than 40% of commercial ships have a level of fouling that combined could produce at least 110 million metric tons of excess carbon emissions and increase fuel costs for the global commercial fleet by $6 billion.

To help find solutions, the US Office of Naval Research (ONR) has funded research into biofouling alternatives for more than 20 years.

“The goal of our program is to influence the development of better coatings,” says Paul Armistead, a program officer in the ONR’s Naval Materials Division. “We want to be environmentally friendly, and we need to be able to carry out our mission around the globe without running into any country’s environmental regulations.” These concerns have become more relevant as the primary antifouling biocides currently used, copper and zinc, have become regulatory targets because of their environmental impact. Yet because these metals already dominate the market, any nonbiocidal alternative must work hard to compete against them.

“Since we work on a more fundamental level, we can try exotic things, like electric fields or piezoelectric approaches” that physically repel marine organisms, Armistead says. “But at some point, we have to sit back and ask: Is this ever going to compete with paint?”

A slippery solution

Barnacles and mussels secrete a strong protein-based glue that helps them stick to underwater surfaces, including docked ship hulls. Seaweed, algae, and various microorganisms can also stick to hulls via thick, viscous biofilms bound together by polysaccharides. Some of the first antifouling treatments that didn’t rely on toxicity were fouling-release coatings made of silicone or polytetrafluoroethylene. Rather than killing organisms, these materials create slippery surfaces that make it difficult for the organisms to adhere in the first place. These materials are readily commercially available today but have not been widely adopted, partly because of the effort required to switch from an antifouling paint to this type of coating. “You might have to sandblast, go back to the bare metal of the hull to build up a new coating system,” University College Cork’s Sullivan says. “And you have the uncertainty of whether this new coating format is going to be as effective as the stuff you’ve been using for years or decades.”

But the biggest issue holding back fouling-release coatings, Sullivan says, is that they don’t prevent biofouling; they just make it easy for shear forces from the boat’s motion to knock any organisms off the hull. These forces don’t work when a boat is stationary. In April, boats expected to be in constant motion suddenly found themselves immobile as the COVID-19 pandemic in the US unexpectedly stranded oil tankers and cargo ships in port. If those ships had used fouling-release coatings, they would have likely built up a large amount of fouling because they hadn’t been moving, Sullivan says. “In those situations, a biocidal coating is working away.”

While immobile ships benefit from biocidal coatings, being stranded in one spot means that leached biocide is concentrated in a specific area, increasing damage to the local ecosystem.

Some researchers have tried using surfaces with microstructural patterns, similar to those of shark skin, to discourage adhesion of marine life even when boats aren’t moving. But the size scale of the patterns that work well against barnacles and mussels is too large to be effective against smaller organisms such as algae. And micropatterning a surface the size of an oil tanker or aircraft carrier would be prohibitively expensive.

The ONR has funded research into such alternatives, but “there’s just too many different things that can come at too many different size scales,” Armistead says. “We still think of it as something that could help, but it’s not going to solve the problem itself.”

Attraction, repulsion

The ideal solution would replace the toxic copper and zinc in marine paints with something nontoxic and affordable. The most promising class of compounds at the moment comes from research into medical devices, which can accumulate biofilms and layers of deposited proteins when implanted in the body. To prevent fouling on medical implants, device makers commonly coat them with polyethylene glycol (PEG), a soft, biocompatible polymer. PEG has recently been incorporated into some fouling-release coatings for ships.

PEG uses a layer of water molecules hydrogen bonded to its surface to block biofilms and proteins from sticking. “But hydrogen bonds are weak and get weaker with heat,” which would be a problem in warm, tropical waters, says Shaoyi Jiang, a chemical and biomedical engineer at Cornell University. So his group tried taking a similar approach but with ionic bonds instead.

For the past 2 decades, the ONR has been funding Jiang’s research on zwitterionic—zwitter being the German word for “hybrid”—antifouling materials. These materials use zwitterions, molecules with equal numbers of positively and negatively charged groups, attached to polymer chains in alternating patterns to build a strong, stable water layer that prevents organisms from adhering to a surface, thus inhibiting biological growth (Acta Biomater. 2016, DOI: 10.1016/j.actbio.2016.03.038). “Because of the strong hydration, our materials look just like water to fouling organisms,” Jiang says. The fundamental mechanism is similar to that of PEG but stabler because of the stronger ionic bonds that form.

Zhan Chen of the University of Michigan and his research group work with Jiang’s team to characterize these materials. In one experiment looking at marine fouling, they pressed mussels against a metal surface coated with poly(sulfobetaine methacrylate) and held them there with a rubber band.

“The mussels tried very, very hard to leave the surface. They really didn’t like it,” Chen says.

Some mussels appeared to give in to their predicament and at last tried to adhere. To the researchers’ surprise, the mussels successfully stuck to the zwitterionic material just as strongly as they would to iron or another metal, such as aluminum.

This response seems to be unique to zwitterionic materials. It’s not that marine organisms can’t stick to the materials; “they just don’t want to,” Chen says. “I think that will be very promising.”

Marine environments are much tougher than medical applications, Jiang says, because there are so many more types of organisms. “It feels like every time you try, you fail on at least one [organism].” With different creatures’ adhesion techniques acting on vastly different size scales and at different temperatures, zwitterionic materials that worked well in Singaporean waters could fail off the coast of New York. But recent versions of these materials that strike the ideal balance between softness, which allows for fouling release, and durability have been more universally successful.

Fundamentally, zwitterionic materials represent a shift in paradigm for antifouling solutions. Rather than killing organisms or using shear forces to push them off, zwitterionic coatings make ship hulls effectively invisible.

After years of trial and error, the commercialization of these materials is in sight, Jiang says. The new materials’ benefits go beyond blocking the necessary organisms: “We can spray our coating with a spray gun; we can make it economical. It’s just a matter of making more.”

Jiang’s material will soon be tested on actual ship hulls rather than small test panels. These experiments will demonstrate whether the zwitterionic coatings can economically compete with biocidal paint and how many years they can last on an active ship. If these tests work, zwitterionic coatings may at last make sailing the ocean blue more environmentally friendly.

Meredith Fore is a freelance writer. A version of this story appeared in ACS Central Science: cenm.ag/antifouling.