Copper in Third-Generation Antifoulants For Marine Coatings

Copper Applications in Industrial & Marine Applications

By William H. Dresher, Ph.D.

Background | The Basis of Antifouling Coatings | Second-Generation Antifoulants |

New copper-based coatings offer improved properties while avoiding environmental hazards associated with tributyltin.

In the face of international bans on the use of organotin (principally tributyltin, TBT) biocides, copper continues to play a major role in antifouling paint systems. Thanks to recent innovations by marine paint and pigment manufacturers, copper-containing TBT-free antifoulants now offer the same premium performance as TBT-based antifoulants while presenting a significantly lower impact on the environment than the now-banned organotin compounds.

Background

Because of copper's biocidal properties, copper and copper compounds have been used for centuries for the protection of the hulls of ships. Over the ages two different purposes have been served in copper-based ship hull protection. In early wooden ships, a sheathing of metallic copper was used to protect the timbers from being invaded by shipworms known as teredos - worms that bore into wood and weaken its structure. Simultaneously, copper-sheathed ships were protected from the growth of barnacles and algae that increased the frictional resistance of the water against the hull and slowed the speed of the ship. In fact, it was claimed that the speed of Admiral Nelson's copper-sheathed ships contributed to the British victory at Trafalgar. Shipworms are not a problem in modern steel, aluminum and plastic ship hulls; however, the growth of friction-causing marine organisms is detrimental. Fouling both reduces the speed of the vessel and increases its fuel consumption. It also increases wear on propulsion systems. In 1915, a Danish manufacturer, J. C. Hempel, invented the first antifouling marine paint and such paints have been the standard of the industry ever since. Thus, today, whether the ship is an ocean-going freighter or tanker, a racing yacht, or a weekend sports boat, antifouling bottom coatings have become a necessity.

The copper compounds that are commonly approved and used in antifoulants for ship bottom protection are: copper (I) oxide, Copper (I) thiocyanate; powdered or flake copper metal; copper bronze (an alloy of copper with tin); copper napthenate; copper resinate (an industry name for the reaction product of copper carbonate hydroxide and rosin); and copper (I) sulfide. Copper pyrithione has been found to be an effective biocide; however, it is not approved by the U.S. EPA and thus is not available in the United States.

Today's antifouling paints typically contain on the order of 40 weight percent of copper compound and are dark brown or dark red in color. A number of specialized copper (I) oxide products are offered for use in various service applications. In addition to standard copper oxide products of the sort that yield the traditional brown or red color, copper (I) oxide products are also available that enables the paint to be pigmented to other colors. These latter products are mainly used in antifoulants for yachts.

Copper is ideal for use as an antifoulant since it is a naturally occurring material that is an essential element required for normal growth by all plants and animals. As such it is a normal constituent in the ecosystem in both soil and water, where its presence is actually in part due to the metabolic by-products of plants and animals. Further, the chemistry of copper can change in the environment, thereby affecting its bioavailability. It is generally agreed that it is the cupric ion, Cu (II), that is mostly responsible for copper's bioactivity. On the other hand, numerous studies have shown that most of the copper present in marine environments is not present as cupric ion. In fact, the concentration of cupric ion in natural waters is an order of magnitude less than the "dissolved" or total copper concentration. Most of the dissolved copper is complexed with organics in both marine and freshwater environments. This complexation reduces or completely inhibits copper's bioavailability and, therefore, its toxicity to marine organisms. Sedimentation removes particulate copper compounds from the water column and introduces it into bottom sediments. A key factor in the environmental aspect of copper-based antifoulants is that the active form, cupric ion, exists only briefly while it is on the surface of the coating. During that time it performs its duty by impeding the settlement of organic life; following which, it is deactivated by reaction with suspended organic material in the water.

The Basis of Antifouling Coatings

Biofouling is most commonly prevented by the use of antifouling paints or coatings. These coatings are of two types: 1) those which present a sufficiently smooth surface to which marine life cannot adhere and 2) those that contain one or more biocides that are released during the lifetime of the coating and which discourage the settlement and growth of marine life. This discussion concerns the latter of these types.

The goal in the design of biocidal antifoulants is to provide a surface that has active biocides present but which either do not leave the surface or are deactivated (rendered non-bioavailable) shortly after leaving the surface. Thus, biocides that have a short active half-life, either by biodegradation or by complexation, are required. There are over 4000 fouling organisms present in the waters of the world. These are classified into hard organisms (such as barnacles) and soft organisms (grasses and algae). Thus, an effective antifouling coating must have a broad spectrum of activity, and this activity must be continuously available to protect the hull against biofouling. In general, copper biocides are most effective against hard fouling; whereas, organic biocides are most effective against soft fouling. Over the years, a number of different system binders and antifouling agents have been developed to meet a variety of usage conditions.

Conventional antifouling systems (we'll refer to these as first-generation systems) rely on the leaching of the antifouling agent from an insoluble film to prevent fouling. In the late 1960s the concept of self-polishing copolymer (SPC) antifoulants was developed. These coatings comprise the so-called second-generation antifoulants. Such coatings contain a cross-linked copolymer in which the cross-linking agent reacts with the sodium ion in seawater, causing it to slowly dissolve away, leaving a fresh concentration of biocide within the surface micro-layer of water adjacent to the paint. This cycle of dissolution and fresh exposure prevents the settlement and growth of juvenile fouling organisms. SPC systems are now the standard of the industry and are offered by a number of marine paint manufacturers.

Second-Generation Antifoulants

Copper, in various chemical compositions and various particle shapes and sizes, was the preeminent inorganic biocide used in antifouling paints until the development of the SPC antifoulants. At that time, TBT was introduced as an active ingredient in antifouling paints. This compound acts both as a cross-linking agent for the polymer matrix of the paint and as a biocide. Copper compounds were retained in most formulations as a pigment in order to fulfill the spectral requirements of biocidal activity needed to accommodate the various forms of marine life encountered. However, because TBT is a very powerful and long lasting biocide, it became the backbone of SPC antifoulant systems. With the persistent biocide, and the successful ablative action offered by the SPC/TBT resin formulation, these systems offered an advantage over conventional copper antifoulant systems. In particular, the TBT systems could last for the five years required by marine insurers between drydockings for bottom inspection and maintenance. First-generation copper-based coatings could last, at most, three years. Thus, coatings using TBT became the industry standard for antifoulants on ship hulls, as well as on pilings, levees and dock structures.

Environmental Impact

Widespread use of TBT-based antifoulants on all vessel types in the commercial and pleasure craft fleets since 1970 has resulted in elevated ambient concentrations of TBT in water samples taken from yachting marinas, busy harbors and waters adjacent to ship repair facilities. High concentrations of TBT in sediments were implicated in damage to cultivated oysters and to populations of coastal dogwhelk, Nucella lapillus (an edible marine snail) that developed imposex characteristics (induction of male characteristics in females) and, in extreme cases, suppression of breeding activity. Further, it was found that TBT-contaminated oysters also tended to store copper; whereas, those without TBT contamination did not.

Consequently, in 1989, TBT-based antifoulants were banned by the European Community for use on vessels less than 25 meters in length. This ban was closely followed by similar proscriptions in the United States, Canada and New Zealand. In 1990, the Marine Environmental Protection Committee (MEPC) of the International Maritime Organization (IMO) called for an international ban on the use of TBT-based antifoulants worldwide for vessels less than 25 meters in length. Japan restricted the application of TBT-based antifoulants on all vessels by shipyards in 1990 and then banned their use completely in 1992. In the mid-1990s, imposex and a reduction in the numbers of the common whelk, Buccinum undatum (a snail that is commonly eaten in Europe) taken from the floor of the North Sea were reported. TBT was implicated as the causative agent. In addition, during this time period, organs removed from a variety of marine mammals showed increased concentrations of TBT, indicating that TBT was bioaccumulative.

As a result, numerous environmental organizations called for a total ban in the use of TBT-based antifoulants. Consequently, in November, 1998, the MEPC-IMO passed a draft assembly resolution calling for a global ban on the application of antifouling products containing organotin compounds by January 1, 2003 and the complete prohibition of their presence on ships' hulls by January 1, 2008.

Third-Generation Antifoulants

The challenge for marine paint manufacturers has been to formulate TBT-free products that perform as well as TBT-based antifoulants but which have a minimal impact on the environment. After more than ten years of research and development, the paint manufacturers have developed effective TBT-free formulations that retain copper as the major biocidal agent.

According to Dr. Julian E. Hunter of International Marine Coatings, Ltd., copper has been found to be safe to humans and to the environment when used in antifouling paints, and is approved for use by most government authorities throughout the world. It is clearly accepted that the use of copper in antifouling paints is significantly safer to the environment than TBT. In fact, in a paper submitted to the MEPC-IMO, the environmental advocacy organization, Friends of the Earth stated, " . . . while naturally-occurring copper is a pollutant, it is considerably less hazardous to the environment than man-made TBT." While the concept that any "naturally-occurring" substance, including copper, is a pollutant is debatable, it is interesting to note that Friends of the Earth also acknowledged that submissions made to the IMO suggest that copper is 1000 times less harmful than TBT. Copper antifoulants are accepted in all national jurisdictions. In Canada, copper antifoulants are acceptable providing they have a release rate of less than 40 µg Cu/cm ²/day and in Sweden that they have a release rate of less than 55 µg Cu/cm ²/day.

Several TBT-free SPC systems have now been developed in what have been referred to as third-generation antifouling coatings. These new coatings all are reported to offer the required five-year lifetime. For example, one marine paint manufacturer has developed and patented an SPC system using copper acrylate as the cross-linking agent in the polymer. Upon reaction with the sodium ion in seawater, the polymer hydrolyzes and the copper acrylate is converted to copper carbonate. This, together with copper (I) oxide pigment and organic booster biocides contained in the formulation, provides the antifouling protection for the required five-year life of the coating, a claim that is borne out in the operating experience of more than 1000 operating vessels.

Several manufacturers offer variations of this formulation that contain a zinc-based cross-linking agent. One such system utilizes zinc carboxylate, while another contains zinc acrylate as the cross-linking agent. Upon contact with seawater, the zinc compound hydrolyzes and zinc is exchanged for sodium in the coating, thus making the coating water-soluble.

In yet another TBT-free SPC system, a silyl cross-linking agent replaces the TBT. (Silyl is an organic radical in which silicon has replaced a carbon atom forming a class of compounds known as silicones) used in a number of products including food additives, artificial organs and childrens' toys.) The system has undergone seven year of testing and is reported to have been applied successfully in 400 dry-dockings and on 130 newly constructed ships. The product is reported to become increasingly smooth as the coating wears, thereby reducing friction as the ship moves through the water.

Yet another manufacturer offers a fiber-composite formulation that, it is claimed, results in enhanced mechanical properties as well as controlled polishing rates along with a constant release of biocides.

In all of these third-generation antifoulants, copper compounds, whether in the polymer or in the pigment, are the primary and essential biocidal agents. Zinc pyrithion and/or organic biocides may be added as "boosters." Third -generation systems also may take the form of paints or epoxy-based plastic coatings. The latter type contains more than 60% by weight of powdered or flaked metallic copper. It is applied to both stationary structures and yachts. The coating has been successfully used, for example, to repel the settlement of zebra mussels on structures in the U.S. Great Lakes. It has also found a use in aluminum-hulled pleasure craft where conventional copper-based antifoulants cannot be used due to the galvanic reaction between the copper compound and the aluminum hull. However, because the coating does not ablate, its surface must be kept clean by power-washing periodically. In addition, it must be abraded with sandpaper every year or two to expose fresh surface.

Summary and Conclusions

New developments in TBT-free coatings have assured a continuing market for copper compounds in marine antifoulants. Widespread acceptance of the new TBT-free antifoulants may not lead to an increase in the amount of copper consumed in these products inasmuch as copper compounds have always been an integral part of the TBT antifoulants that the new products replace. However, because of ecological problems associated with organic biocides, copper-based antifoulants are expected to continue to provide clear advantages in the protection of the environment. And, in addition to the ecological advantages of the new copper-based antifoulants over TBT and organic biocides, their ability to provide self-polishing properties while maintaining a fouling-free surface will result in better operating efficiency, lower fuel consumption and reduced emission of greenhouse gases.

Also in this Issue:

• A New Copper Roof for the Bodleian Library in Oxford
• Copper in Third-Generation Antifoulants For Marine Coatings

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