Advances in antifouling

Antifoulants in the commercial sector (see page 53) might seem of little consequence to pleasure-boat owners. After all, for years now, it’s been illegal to apply tin-based paints to most yachts (the exceptions being aluminum craft and vessels over 82 feet). However, this perspective fails to take into account the vital importance of market volume in fostering research and development activity. Despite steady, worldwide growth in private yacht ownership, the commercial demand for antifoulants will always be overwhelmingly greater. Moreover, fuel costs, dry-docking and down time represent a much larger proportion of the operating budget for a commercial ship as opposed to a pleasure yacht, so shipping firms are quite willing to pay top dollar for coating systems that deliver long-lasting antifouling performance.

But by the same token, effective tin-free antifouling, although developed for large commercial vessels, may well be of special relevance to the voyaging sailor. Unlike coastal yacht owners (who typically haul out annually for winter storage), many voyagers would much prefer to remain afloat for years on end between haulouts. Time will tell whether the latest formulations will again make multiyear antifouling a viable proposition (as it was in the 1980s when tin-based ablatives reigned supreme). However, it seems certain that the extensive product refinement and documentation demanded by the commercial sector will have a positive impact on the antifouling products available on the yachting market.

New copolymer paints

The principle behind ablative bottom paints is straightforward but tricky in practice. The ideal formulation will self-polish at a slow, constant rate whether the vessel is underway or at rest. As it does so, it continually exposes just enough fresh biocide to deter marine growth. Environmental concerns aside, tributyltin (TBT) was ideal for ablative paints, not only due to its broad toxicity, but because it readily combined with common paint vehicles to form what’s widely described as a co-polymer — a reasonably durable paint that nevertheless wears away at a gradual, uniform rate when submerged in seawater.

Overwhelmingly, the dominant toxicant in the new generation of self-polishing antifouling paints is the old standby copper oxide. Many are also fortified with a “booster biocide” such as zinc oxide, and copper or zinc pyrithione. For use on aluminum hulls, copper thiocyanate is sometimes favored because it’s less prone to promoting galvanic hull corrosion. None of these biocides is as effective against algal growth as TBT was, so it’s now common practice to include a specific, high-potency algacide such as Irgarol produced by Ciba-Geigy.

The paint binder that carries these toxicants in a modern co-polymer product is usually a complex acrylic formulation, and the desired self-polishing behavior results from a hydrolytic chemical reaction — not simply erosion due to water flow. All the major marine paint suppliers — including Akzo Nobel (International), Hempel, Jotun, Sigma Coatings and others — now offer comparable products.

By and large, the one major drawback of these advanced, tin-free bottom paints is cost &mdash typically almost $200 per gallon at the retail level.

Metallic copper in various guises

Back in the days when wooden ships were clad with copper plates, this straightforward approach to antifouling was the norm. Today, however, it’s a rarity, although optimistic inventors regularly come forward, hoping to launch a revival.

In 1971, the U.S. copper industry sponsored the construction of a 20-meter Gulf Coast shrimp boat made of copper/nickel alloy (90 percent Cu, 10 percent Ni). The result was a rugged, welded hull with permanent, built-in antifouling, but the high cost of this metal has discouraged widespread use. Likewise, numerous attempts to develop techniques for permanently bonding Cu-Ni foil to the underbellies of boats have met with limited success &mdash typically because the adhesives used proved to be less than permanent in the marine environment. Nevertheless, the copper-foil vision keeps resurfacing (so to speak), and sooner or later someone will probably get it right.

Fine copper powder or flake can be blended with epoxies or other thermosetting resins to produce a coating with reasonable antifouling properties (although the bulk of the copper remains encapsulated in waterproof plastic). Probably the best-known of these products is CopperPoxy, manufactured by American Marine Coatings in Seattle. Back in 1992, I applied an early version CopperPoxy to my 30-foot sailboat, and through seven years of continuous immersion, I was reasonably satisfied with the results. Over this time frame, the antifouling properties diminished considerably, necessitating in-water scrubbing at increasingly frequent intervals. Nevertheless, the oxidized copper surface remained surprisingly effective in preventing barnacles from getting a firm grip. Since 1999, I’ve twice applied single coats of a copper-based ablative paint over the CopperPoxy, operating on the theory that the underlayer will continue to resist fouling to some extent as the paint wears thin. So far it’s been working pretty well.

Nonstick surfaces

Lately, there’s been growing interest in foul-release coatings as a benign alternative to conventional antifouling agents containing noxious constituents of any kind. The basic idea is to achieve a surface so slick that marine fouling organisms are unable to get a secure grip. Teflon-containing formulations have been around for a long time, but none has so far lived up to the slippery reputation.

More promising are the silicone-based coatings &mdash slick, rubbery materials that resist wetting (exceptionally low surface energy).

Current silicone nonstick coatings don’t stop fouling organisms from settling, but they do prevent them from adhering strongly enough to remain attached when exposed to moderately brisk flow or even the mildest mechanical wiping. Eight years ago, I submerged a fiberglass sample panel coated on one side with a silicone product called Veridian &mdash International Paint’s first attempt to market a product of this type. Since then, massive fouling from the untreated side of the test panel has repeatedly spread and overgrown the coated face. However, everything sloughs off with a quick swipe of the fingers, leaving the silicone surface looking as good as new. It’s an impressive demonstration, yet Veridian failed to generate much enthusiasm within the yachting community, in part due to high cost (at least $35 per square foot) and because the soft, rubbery coating is more susceptible to mechanical damage than conventional bottom paints. Also, without much demand for Veridian, yards were reluctant to apply the stuff because once spray equipment is exposed to silicones, it can never be cleaned well enough for other applications.

So at present, Akzo Nobel is focusing on the big-ship applications for a newer generation of silicone foul-release products, now sold under the Intersleek banner. Operating speeds greater than 15 knots are preferred to ensure temporary growth is dislodged. However, although few voyaging yachts ever achieve such speeds, my (admittedly limited) experience with Veridian suggests that wiping down a silicone-coated bottom should be about the easiest boat-cleaning job a diver will ever see.

In addition to Akzo Nobel, General Electric, Hempel and others are offering silicone bottom treatments; while more advanced fouling-release technologies may well be on the way. For example, a recent item appearing in Environmental Health Perspectives (by Scott Fields, July 2003) describes an experimental bi-layer system developed at Cornell University. It consisted of a tough, synthetic rubber base layer and a nontoxic topcoat of specialized liquid-crystal materials that effectively block marine growth from adhering. Further field testing is currently underway.

Some unusual approaches

Both swimming and bottom-dwelling marine organisms are themselves vulnerable to fouling, and many use biochemical weapons to repel unwanted settlers. Among the most effective of these natural defenses are enzymes that attack the bio-adhesives secreted by fouling organisms. And because enzymes function by catalyzing chemical reactions (rather than being consumed as the reaction progresses) &mdash in principle a little can go a very long way. Just as medically valuable, antibiotics have been isolated from marine organisms and subsequently synthesized in useful quantities, there’s an excellent chance that naturally occurring antifouling enzymes can also be replicated for commercial use. Indeed, the two classes of biochemicals fulfill distinctly similar roles in the lives of the organisms that produce them.

Enzyme-based antifouling technologies are still in the developmental stages, but the E Paint Co. of Massachusetts has, for the past dozen years, been marketing a successful antifouling paint that releases no environmental toxins. Instead, a photochemical reaction at the surface of the paint film generates small quantities of hydrogen peroxide &mdash a reactive chemical often used as an antiseptic. The peroxide deters the settlement of marine larvae but quickly breaks down into harmless water and oxygen. Several water- and solvent-based formulations are available. Antifouling effects are said to last one season.

Taking a very different tack, Swedish inventor, Kjell Alm, has devised a unique nontoxic treatment that is claimed to effectively deter hard fouling organisms. The application involves blowing extremely short synthetic fibers onto a surface precoated with wet epoxy. During the process, an electrostatic charge is applied to the microfibers, causing each to land upright with one end in the epoxy and the other extending vertically. For obvious reasons, this artificial velvet finish is being marketed as SealCoat. It will be interesting to see how it performs over time and in various fouling environments.

Choosing antifouling

Change comes slowly to the marine community, and there’s little danger that most traditional antifouling remedies will disappear from the market anytime soon. Indeed, at least one of the smaller U.S. paint manufacturers is currently doing a roaring business, legally exporting tin-based products to the Caribbean, where, in many cases, they end up on the undersides of U.S.-flagged vessels. Hopefully, this practice will not continue indefinitely, but it certainly points out the ongoing tension between expedience and environmental responsibility.

Practical considerations are undeniably important when it comes to selecting an antifouling treatment. If the vessel is already in service, it will likely be necessary to select a product that’s compatible with the old bottom paint that’s already on the hull. Ablative paints are generally more expensive than hard paints but may prove less costly and troublesome over time because they eliminate the necessity of stripping off heavy accumulations of “dead” paint after a number of years.

Antifouling paints that release their biocide content in a gradual, uniform manner are much superior to those that unleash a heavy dose of toxins immediately after launching but taper back to much lower release rates after a fairly short time. For this reason, the bottom paint with the highest percentage of active biocides may not always be the best-performing product.

Other important considerations include minimum drying time and the maximum time a freshly painted hull can remain out of water without degrading the antifouling properties.

When it comes to the new, benign treatments, such as nonstick treatments and artificial fur, it may be difficult to secure unbiased evidence of product efficacy. Hopefully, these novel approaches will prove themselves over time, because there’s little question that any toxic chemicals released into the sea in quantity have the potential for environmental damage. n

Contributing Editor Sven Donaldson, a former sailmaker and current marine technical writer is based in British Columbia.

By Ocean Navigator