Voices
Antifouling Paints and the Environment
BY DR. J. E. HUNTER AND DR. C. D. ANDERSON
INTRODUCTION The need to prevent fouling on ship’s hulls is undisputed and universally recognised as critically important for efficient commerce and to minimise the environmental impact of shipping. If the world fleet were unprotected against fouling, it has been calculated that an extra 70.6 million tonnes of valuable fossil fuel would be burned each year, leading to an extra 201 million tonnes of carbon dioxide and 5.6 million tonnes sulphur dioxide gas emitted into the atmosphere. Ship fouling is most commonly prevented by the use of antifouling paints. Antifouling paints contain biocides that are released during the lifetime of the coating, creating a concentration of biocide within a surface micro-layer of water adjacent to the paint surface, preventing settlement of juvenile fouling organisms. There are over 4000 marine fouling species, therefore biocides used in antifouling paints must have a wide spectrum of activity to cover such a diversity of species able to colonise a ship’s hull.
Due to environmental concerns over the use of TBT as an antifouling biocide and general disquiet over the release of biocides into the environment, new and evolving regulations now demand that antifouling paints must not result in adverse effects on the environment. This has resulted in a significant challenge for the coatings industry, and developments have been underway for many years to emulate the efficiency of antifouling paints based on TBT, whilst also having significantly reduced environmental impact.
This paper summarises developments in TBT-free self polishing co-polymer antifouling paints. The future for antifouling paints is also discussed, in light of environmental and regulatory scrutiny currently underway throughout the world and within the Marine Environmental Protection Committee of the IMO (IMO-MEPC).
TBT-SELF POLISHING COPOLYMER ANTIFOULING PAINTS
The "industry standard" to which the performance of antifouling paints is measured is that proven with TBT (tributyltin) self-polishing copolymer (TBT-SPC) products. TBT-SPC products contain an acrylic copolymer as the film-forming component of the paint, with TBT biocide chemically bonded to it. In the final paint product other biocides, including copper compounds and ‘booster biocides’ are added to the TBT polymer along with additives and solvent. On immersion a reaction occurs between sea-water and the TBT copolymer, releasing biocidal TBT and the other biocides present. The reaction is confined to the top few nanometres of the paint surface and, in addition to releasing biocide, results in formation of a water-soluble reaction product causing the paint surface to ‘polish’ with time as the vessel moves through the water. As the mechanism for biocide release is via a chemical reaction, it can be finely tuned to maximise the lifetime of TBT-SPC systems, with antifouling lifetime directly related to the thickness of coating applied. Thus in-service lifetimes of up to five years can be specified. As the paint film is acrylic in nature, TBT-SPC products are mechanically strong and able to withstand extreme environmental conditions such as those experienced in boot-top areas and long periods of exposure to the atmosphere as is the case at new construction.
The wide spectrum of biocidal activity shown by TBT against fouling organisms, coupled with a highly controlled biocide release rate, allows vessels coated with TBT-SPC products to maintain a high level of fouling control for up to five years.
Typical records of high performance TBT-SPC products show that up to 96% of vessels return to dry-dock in a satisfactory condition at the end of the specified lifetime of the paint scheme applied.
ENVIRONMENTAL IMPACT OF ANTIFOULING PAINTS
Widespread use of TBT-SPC products on all vessel types in the commercial and pleasure craft fleets since the 1970s resulted in reports of elevated ambient concentrations of TBT in water taken from yachting marinas, in busy harbours, and in areas close to ship repair activity. These localised concentrations of TBT, particularly in sediments, were implicated as responsible for damage to cultivated oysters in the vicinity of marinas and to populations of the coastal dogwhelk Nucella lapillus, which showed imposex (induction of male characteristics in the females) and suppression of breeding activity in extreme cases. In response, the use of TBT on vessels less than 25 metres length overall was banned in 1989 throughout the European Union, with similar bans introduced in USA, Canada, Australia and New Zealand.
In 1990, the IMO-MEPC issued a series of recommendations on the use of TBT antifoulings, including a ban on its use on vessels less than 25 metres LOA, a call for reduction in discharges of TBT from ship yards, and for a maximum TBT release rate of four micrograms TBT /square cm /day to be established for all antifouling paints. Japan went further by first restricting applications of TBT-SPC in Japanese shipyards (1990) and subsequently banning all applications in 1992. In the mid 1990s imposex and a reduction in the numbers of the common whelk Buccinum undatum, taken from the floor of the North Sea, were reported and TBT implicated as the causative agent. In addition recent analysis of organs removed from sea mammals, fish and some birds have also revealed small but detectable concentrations of TBT. This latest data, although not conclusively demonstrating that TBT is impacting on these species, has caused several governments and environmental organisations to call for the use of TBT as an antifouling biocide to be totally banned throughout the world.
In calling for a ban, many governments cited the precautionary approach, as agreed at the United Nations Rio Convention (and as agreed by IMO-MEPC), as justification to call for a global ban of TBT. This approach permits regulatory action to be taken if there is scientific concern that the use of a material is unsafe to the environment, without the need for scientific proof that actual effects are occurring. In response to calls from several governments to ban the use of TBT as an antifouling biocide throughout the world, the 42nd meeting of the IMO-MEPC (November 1998) unanimously passed a ‘draft assembly resolution’ calling for a global ban on the application of antifouling products containing organotin compounds which act as biocides by January 1, 2003 and a complete prohibition of their presence on ships’ hulls by January 1, 2008.
The antifouling debate continued at the 43rd meeting of the IMO-MEPC in June, 1999 with MEPC delegates agreeing that a TBT ban is best achieved by adoption of a ‘free-standing’, globally-enforceable IMO convention specific for antifouling products. The new convention, a draft of which was presented by the USA delegation to IMO-MEPC43, will be independent of existing IMO conventions, and will also contain a mechanism to ban other products in future if they are deemed unacceptable to the marine environment. A major work item ahead is establishing the ‘entry-into-force conditions’ of the new convention. Critical in the new convention is the adoption of a risk-based scientific mechanism (based on the precautionary principle as adopted by IMO-MEPC) to evaluate alternatives to TBT nominated for review in future. The possibility of alternative products generating similar environmental concerns to those of TBT seems remote, but clearly there must be a mechanism to ban proven environmentally damaging materials if they appear in futre. The draft IMO assembly resolution was discussed and ratified in November, 1999 at the IMO assembly. IMO-MEPC will need to further discuss how the ban will work and be legally enforceable at the international level. A key issue which must be resolved by MEPC is the meaning of ‘a complete prohibition’ of the presence of TBT products on ships’ hulls by 2008. Does this mean all vessels with residual (spent) TBT-SPC paint on the hull must be completely blasted back to the steel hull by 2008? Or will a sealer coat applied over any residual TBT antifouling on the hull suffice to prevent any releases to the environment? This is a key issue for IMO-MEPC to resolve as it has an impact on the choice of antifouling products on vessels at new construction and maintenance and repair projects today [see sidebar on page 15].
TBT-FREE SELF POLISHING COPOLYMER ANTIFOULING PAINTS
The challenge for paint manufacturers has been to formulate products which perform as well as TBT-SPCs but which have minimal impact in the marine environment. This has been achieved with biocidal antifoulings by : i. Use of an acrylic polymer that does not contain TBT but which reacts with sea-water in the same controlled way as TBT-copolymer; and ii. Using biocides which can exert their activity at the paint surface but have minimal environmental impact. Intersmooth Ecoloflex SPC antifoulings contain a novel self-polishing copper arcylate polymer which reacts with sea-water in a reaction analogous to TBT-SPC copolymers. This produces a soluble micro-layer at the paint surface, causing the paint surface to ‘polish’ with time. Copper oxide and a non-persistent, degradable ‘boosting biocide’ are physically dispersed into the polymer solution, to provide the biocidal activity of the paint. As the polymer at the paint surface hydrolyses in water, the biocides are released in a finely controlled fashion, creating a concentration of biocide at the paint surface, preventing fouling. Once the biocides have diffused away from the paint surface, the copper oxide quickly detoxifies and the ‘boosting biocide’ degrades in the presence of light and/or bacteria into substances with very low toxicity and persistence.
ENVIRONMENTAL IMPACT OF BIOCIDES IN SPC PRODUCTS
The biocides in SPC products, Intersmooth Ecoloflex for example, are copper oxide and zinc pyrithione. Copper is safe to man and the environment when used in antifouling paints for ships and is registered for use as an antifouling biocide in products for commercial vessels with government authorities around the world. It is clearly accepted that its use in antifouling paints is much safer to the environment than TBT. In a paper submitted to the Marine Environmental Protection Committee of IMO4 (IMO-MEPC40) the Friends of the Earth organisation stated "while naturally occurring copper is a pollutant, it is considerably less hazardous than man-made, persistent organic pollutants like TBT". They also noted that submissions made by delegations to IMO suggest that copper is 1000 times less harmful than TBT.
Zinc pyrithione is degraded and becomes non-toxic quickly after release from the coating. It is not persistent and will result only in minimal impact on the marine environment. The other major use of zinc pyrithione is as an anti-dandruff agent in shampoos. As many thousands of tonnes of this substance is applied to human hair each morning and subsequently discharged into the environment via sewage treatment works without incident, its safety to humans and the environment is already proven.
’NON-STICK’ FOUL-RELEASE COATINGS
From an environmental perspective the most desirable approach to fouling control is one which does not rely on the release of biocides to achieve its effect. For high speed specialist vessels (such as fast ferries trading at over 30 knots) and high activity, deep-sea scheduled ships which operate at speeds between 15 and 30 knots, biocide-free fouling control coatings, based on silicon elastomer technology and working on a "non-stick" principle, provide further TBT-free alternatives. The Intersleek coatings do not contain copper and therefore are also ideally suited for use on aluminum hulls. Intersleek 425 is suitable only for high speed (over 30 knots) vessels such as fast ferries. Intersleek 700 is designed for use only on high activity deep sea scheduled ships operating in the 15 to 30 knots range. The vast majority of the world fleet (eg. crude cil tankers, chemical/product tankers and bulk carriers) are not suitable for current ‘foul-release’ technology. For these bulk cargo ships, vessel activity is usually relatively low, such that fouling attachment can easily occur on current biocide-free systems. In addition, the in-service speeds of these vessel types are typically too slow for self-cleaning of fouling that attaches during stationary or slow moving periods. These ships therefore require the biocidal antifouling protection provided by the self-polishing copolymers.