| Marine anti-fouling technologies |
ANTI-FOULING PAINTS
Review papers on anti-fouling technologies
The prevention of the attachment of barnacles, mussels and algae to the underwater surfaces of ships has been an age old problem of the maritime world. In the fourth century BC, Aristotle credited small fish (barnacles) with the ability to slow down ships [1]. Saroyan [2] suggested that fouling growths caused loss of ship speed, increased friction and over-consumption of fuel, promotion of corrosion by mechanical damage, increased weight, prevented operation of moving parts of equipment, reduction of the size of conduit openings and increased noise in sonar signals.
“The Remora is a fish of an ashen hue; on the top of its head it has a cartilaginous disc with which it creates a vacuum that enables it to cling to other underwater creatures. Here is Pliny’s acclamation of its powers [Pliny the Elder, The Natural History, Book IX, Chapter 41]: There is a quite small fish that frequents rocks, called the sucking-fish. This is believed to make ships go more slowly by sticking to their hulls“. “Pliny then goes on to describe the murex, a variety of purple fish also credited with bringing ships under full sail to a standstill: ‘. . . it is a foot long and four inches wide, and hinders ships”. Jorge Luis Borges with Margarita Guerrero, The Book of Imaginary Beings ~ revised, enlarged and translated by Norman Thomas di Giovanni in collaboration with the author, Penguin Books, Harmondsworth, 1974 |
In 1975, the US Navy [3] reported that barnacles and other marine encrustations on hulls can increase drag, slow the vessel down and estimated that "25 per cent of the fuel used by a naval vessel is burnt to overcome this extra drag and weight, adding some $70 million to the Navy's annual bill". The US Naval Surface Warfare Center at Carderock has estimated that biofouling reduces vessel speed by up to 10% with the added drag increasing fuel consumption by up to 40% [4]. Champ [5] reported the potential for annual fuel cost savings in the range US$100-130 million/year for the US Navy in calendar year 2000 plus significant reductions in maintenance costs using TBT anti-fouling (now banned). Abbott et al [6] analysed the cost of a one-way voyage from San Francisco to Yokohama: journey costs for the fouled ship increased by 77% (US$78,300) relative to a newly anti-fouled vessel (the 15 000 t dead weight Greek dry-cargo vessel MV Macedonia) because the economics limited the fouled ship to 8.5 knots (vs 14 knots for the pristine hull) with increased journey distances, avoiding the great circle northern latitudes, required for safe operation at the slower speed. Schultz et al [7] reported increases in fuel consumption of 20.4% (US$2.3M/ship/year) for naval surface ships and estimated the overall cost associated with hull fouling for the coating, cleaning and fouling at US$56M/year for the entire DDG-51 class ($1B over 15 years). The aquaculture industry spends ~US$1.5-3.0 billion/year on antiffoulants or mitigation associated with fouling [8].
Qiu et al [9] have published a comprehensive review of functional polymer materials for marine biofouling control. They divide the options into the following categories:
Toxic compositions (tri-butyl tin, cuprous oxide)
In 1824, Sir Humphrey Davy stated that the anti-fouling action of copper sheathing on wooden ships was related to its rate of solution in seawater [1]. Copper compounds are still generally regarded as one of the best antifouling agents. They are normally applied as cuprous oxide (Cu2O) or cuprous thiocyanate in a paint film.
In the 1960's, the shipping industry began using paints containing an organo-tin compound called tributyltin (TBT) to prevent the fouling of marine surfaces. This chemical act as a highly toxic material that kills larvae and other sea life that comes into contact with the ships. By the 1980's it became evident that TBT is responsible for a host of environmental problems resulting from persistence of these compounds in the sediments and killing sea life besides those attaching to ships. TBT was soon found to cause deformations in oysters, sex changes in whelks and possible toxic effects in higher organisms as TBT enters the food chain. The International Maritime Organization (IMO), an agency of the United Nations responsible for ship safety and maritime pollution, adopted a Resolution in 1990 to urge governments to eliminate TBT in antifouling paints. In 1998, the IMO approved another Resolution for the global prohibition of the application of TBT as biocide in antifouling paints on ships by January 1, 2003 and the International Convention on the Control of Harmful Anti-fouling Systems on Ships imposed a complete ban on the presence of TBT antifouling paints on all ships (including fixed and floating platforms, floating storage units/FSUs, and Floating Production Storage and Offtake units/FPSOs) by 1 January 2008. For a chronology of TBT as an antifouling coating see the MST326 page on the Precautionary Principle.
Turner [10] has reviewed the likely biogeochemical pathways for toxins from anti-fouling paint particles generated during the maintenance of recreational boats, abandoned ships and grounded vessels.
ePaint.com LLC introduced an antifouling coating which relies on a combination of visible light, oxygenated water and a catalyst in the coating to generate a steady source of hydrogen peroxide (H2O2) [11]. After release, the H2O2 is rapidly decomposed to oxygen and water molecules. The H2O2 is effective against hard fouling agents (e.g. barnacles, mussels and tubeworms) and is complemented by zinc pyrithione to combat soft fouling agents (e.g. algae).
Pinori et al [12] at the University of Gothenburg and the SP Technical Research Institute of Sweden in Borås have proposed a new eco-friendly method to fight the accumulation of barnacles on the hulls of boats and ships. Most marine organisms that attach to vessels (e.g. mussels and algae) can be easily scraped off. Barnacles penetrate the surface, so the new technique holds the poison inside the paint, minimising release into the water, until the barnacles penetrate the surface and the toxin is released. The toxin used is the ivermectin molecule produced by the Streptomyces avermitilis bacterium. A concentration of only 1 part per thousand (0.1%) is effective (one gram of ivermectin per litre of paint) and lasts for many years.
Non-toxic low surface energy compositions
These coatings have low surface energy and as a consequence fouling organisms find it difficult to adhere to the surface. Such coatings are not normally suitable for drying moorings. A major benefit is low water resistance on the hull, leading to improved fuel economy and more responsive handling. Swain [13] noted that there is no true non-stick marine coating: the current technologies use fouling followed by release. He found that successful surfaces reduced adhesion below the level where the force of water to release the fouling was 34 kPa (5 psi), while the best coatings had adhesion factors below 7-14 kPa (1-2 psi). Silicone polymers are the normal choice as there is no known environmental damage from these coatings. However, they are relatively expensive, weaker and more vulnerable to damage and their effectiveness may be compromised by improper application (the correct environment is the key to success).
Exfoliating/self-polishing surfaces
As the name suggests, the flow of water over the hull continually erodes the surface of the coating exposing a fresh layer of biocide. A thick coating can provide multi-season performance subject to an annual check and revival of the coating by abrasion/water-jetting. Such coatings are not normally suitable for drying moorings. Hoare [14] and Thompson et al [15, 16] have raised the issue of the final destination of plastic particles that enter the marine environment.
Polymer "brush" coatings
Yang et al [17] have recently reviewd a variety of functional "brushes" and coatings developed to combat marine biofouling and biocorrosion while minimising environmental burdens arising from the systems. The surface-tethered brush polymer can be engineered with specific functionalities. For the inhibition of marine biofouling, the systems have specific fouling-resistant (prevention of attachment), fouling-releasing (reduced adhesion stength) and organism dissuasion (degrading or killing) actions designed to provide a non-biocide releasing, environmentally benign surface. For prevention of attachment, hydrophilic polymers (e.g. poly(ethyleneglycol) a.k.a. PEG) have been shown to resist protein absorption and cell adhesion. For reduced adhesion strength, fluoropolymers and silicones minimise the adhesion strength and allow easy release of the organism. To dissuade settlement, antimicrobial polymers containing cations compromise microbes by disrupting cellular membranes. it is possible to combine these actions in a single coating.
Bio-inspired approach (biomimetics)
Liedert and Kesel [18] sought to verify the efficiency of biologically inspired surface microstructures as an alternative to biocide paints using shark skin as the analogue. The highest antifouling performance (about 95% reduction of settlement of barnacles) was achieved by a combination of the following parameters:
while in a separate study Kesel and Liedert [19] reported that the best antifouling performance (67% reduction of barnacle settlement) was observed on soft silicone surfaces with microtopographies between 40 μm and 2 mm.
Schumacher et al [20] have developed a biomimetic design, Sharklet AFTM microtopographical surfaces, inspired by the structure of shark scales which reduces the settlement of algae spores by over 60% when the characteristic dimension of the surface is 4.2μm or less.
Sullivan and Regan [21] produced synthetic sharkskin samples using a silicone elastomer (Dow Corning Sylgard 184 PDMSe polydimethylsiloxane) and a slow swimming catshark Scyliorhinus caicula as the template. In comparison to a smooth planar surface of identical elastomer, contamination rates were reduced with smaller, densely packed denticles attracting least fouling.
The development of fouling will normally start with micro-organisms forming a film on the surface which then procvides attachement points for other plants and animals. A joint NPL/University of Bristol project engineered biocompatible surfaces exhibiting nanowire arrays. The nanowires, which mimic bactericidal nanoscale pillar structures in the cicada, act as tiny spears that pierce bacterial cells causing their leakage and death. While the project was aimed at guiding mammalian cell proliferation, there may be scope for development in marine antifouling [22].
ANTI-FOULING SYSTEMS
Cao et al [23] have recently reveiewed the full range of anti-fouling technologies. In addition to surface coating technologies, they identify anti-fouling by (a) bubbles created by electrolysis of seawater, (b) vibration, (c) magnetic fields, (d) ultraviolet radiation, (e) radioactivity and (f) modification of surfaces including roughness, topography, hydrophobicity and lubricity. Myan et al [24] have reviewed the effect of surface topography at various scales and geometry on the interaction of marine fouling organisms with submerged surfaces.
Calder [26] found "promise in ultrasound antifouling technology as a suplement to traditional copper paint". The systems typically use up to four ~50 W peak transducers cycling through the frequency range 20-100 kHz. It may be appropriate to apply the precautionary principle to this technology until the effect on marine mammals is established [26].
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Further reading