Modern approaches to marine antifouling coatings

A review of: Modern approaches to marine antifouling coatings. 2006. Chambers, Stokes, Walsh and Wood. Surface and coatings technology, 201, pp. 3642- 3652.

Antifouling systems are required wherever unwanted growth of biological organisms occurs. Paint coatings on ships are used for a wide variety of functions such as corrosion resistance, ease of maintenance, appearance, non-slip surfaces on decking as well as the prevention of fouling on the hull by unwanted marine organisms, which is often prolific as vessels move between a diverse range of environments. The settlement and accumulation of marine organisms on an inanimate substrates can cause large penalties to engineered structures, such as ships hulls by increasing the hydrodynamic drag, lower the maneuverability of the vessel and increase the fuel consumption, leading to increased costs through the increased use of manpower, fuel, material and dry docking time.

The use of toxic antifoulants on ships hulls has been a historic method of controlling fouling but biocides such as lead, arsenic, mercury and their organic derivatives have been banned due to the environmental risk they pose, with recent research focusing on environmentally acceptable alternatives.

The ban of TBT in 2003 created a gap in the market resulting in the current use of other metallic species such as copper and zinc as substitutes, delivered in a modified self-polishing copolymer (SPC) delivery mechanism which uses both hydrolysis and erosion to control the antifouling activity. However, as well as the recent increased skepticism over the use of copper, an increased tolerance has been reported for a select group of macrophytes, including key fouling algal species Enteromorpha (now Ulva). As a result, booster biocides have been incorporated to increase the length and functionality of copper-based antifouling coating systems. Terrestrial pesticides have also been adapted for marine antifouling systems but have increasingly had issues with their persistence and toxicity. This approach is often too species specific or too broad, influencing non-target organisms.

Foul release coating (FRCs) function due to low surface energy which degrades an organism’s ability to generate a strong interfacial bond with the surface. The smoothness of the coating at the molecular level allows for organisms to be dislodged once the vessel is moving beyond a critical velocity. The purely physical effects of these low energy coating provide a unique approach to developing an environmentally acceptable alternative to biocide-based antifoulants. It offers a broad spectrum antifoulant without incurring the issues of biodegradation, legislative standards and fees necessary to register an active antifouling compound. However, this approach does not tackle biofouling while the vessel is berthed dockside – biological communities are allowed to establish and macrofoulers can then be translocated biogeographically causing environmental issues of alien species transport.

Within the marine ecosystem, evolution has allowed for the development of certain antifouling properties: marine organisms have both chemical and physical methods to protect themselves from the harmful process of biofouling. These diverse mechanisms have been investigated in several phyla including Porifera, Echinodermata, Tunicate, Algae, Cnidaria, Bryozoans and Bacteria, with particular interest in the chemical defenses, yielding a variety of potential compounds. However their incorporation into a functioning system to resist biofouling has yet to occur.

Present modern methods of biofouling control are effective alternatives to the TBT antifouling coatings, but not yet their equal. Therefore, research into varied approaches to the design and implementation of antifouling coating technology must continue.