TBT-based antifouling paints

Copper-based antifouling paints


For over thirty years TBT was the active agent in antifouling paints used extensively in the maritime sector. It has been described as one of the most harmful substances knowingly introduced onto the marine environment. In the 1980s it became apparent that the use of TBT was causing severe damage to non-target species in the wider marine environment, such as deformities in shellfish and mollusc communities, reduced growth of algae and toxic effects in young fish. The effects of TBT were particularly noticeable on dog whelk populations near harbours and marinas where female dog whelks developed into males (Loretto and Proud 1993).

Recognition of the widespread environmental affects caused by TBT resulted in the Europe-wide ban of its use in 1987 on boats under 25 metres. At present in the UK the use of TBT-based paints continues on larger vessels and it remains at present the most-effective means of controlling fouling. However, in November 1998 the IMO made the decision to introduce a world-wide ban in the use of TBT in antifouling paints for most ships from January 2003, a ban which has been in place for several years in some countries, such as Japan.

Pressure for a complete ban of the use of TBT in antifouling paints has been increasing with evidence that it is bio-accumulating in food chains, with particularly high levels being found in marine mammals (Iwata et al 1995). The reported effects of TBT in marine mammals include suppression of the immune system. Marine mammals (porpoise and grey seals) stranded along the coasts of England and Wales have been shown to be contaminated with low levels of butyl tin compounds. Whilst the levels of these tin compounds are lower than some of those reported for small cetaceans from other areas, such as Japan, the USA and the Adriatic Sea, further study is required of possible toxic effects of these compounds and the risk their accumulation poses to marine mammals in the UK (Law et al 1998).

It is recognised the IMO ban will need to be gradually introduced and its success depends upon the development of effective substitute paints. Paint manufacturers have been researching and developing alternative paints for some years, with varying degrees of success. At present copper antifouling paints present the best practical environmental option for a TBT alternative available to the marine industry. Phasing out TBT would be undermined if other paints were found to be more detrimental to the environment. The International Chamber of Shipping has stated that the antifouling coatings industry "seems to be seeing a way through the problem" (ENDS Report 1998), although they have warned that a full-scale switch would be at significant cost to the maritime industry. The IMO urges member states to encourage the use of alternative antifouling paint systems, pending the mandatory ban, and to set a timetable for the phasing out of TBT.

Copper-based antifouling paints

The ban of the use of TBT on smaller vessels has resulted in the shift back to the use of copper as the main biocide in the UK. Although copper is a naturally occurring element which is essential for metabolic processes in living organisms, it is also a widespread pollutant in industrial waters which can be one of the most poisonous heavy metals when present in excess. The main sources of copper contamination in the marine environment are from industrial discharges and atmospheric deposition, particularly from foundries and metal processing operations. Fungicides, wood preservatives and boat antifouling paints can also contribute to high levels of copper in the aquatic environment.

In general 95% of the UK recreational market is using some form of copper-based paint (UK CEED 1993). In a study commissioned by the Environment Agency, WRc estimated the amount of copper used on coastal leisure craft in the UK in one year was between 75,173 and 311,769 kg (Boxall, Conrad & Reed 1998). This study found the majority of copper in antifouling enters the marine environment through leaching, and that only a small proportion enters during the removal of antifouling paint, which occurs mostly by water blasting. However, the concentrated nature of the biocide in scrapings and cleaning residues may cause more of a localised environmental problem.

In addition to the widespread use of copper-based paints on leisure boats, they have also been tested on ocean going ships over 25m, particularly in the USA and Japan. There are potential drawbacks of the use copper-based paints, including an incompatibility with aluminium-hulled craft and the production of offensive odours. A new form of copper antifouling developed is the copper based gel coat, or epoxy, that is used widely in the United States. It is claimed that it lasts up to 15 years, but the cost is higher than the previous types of paint and it does not work as well. Certain fouling organisms are resistant to copper-based paints and they have now been supplemented by additional biocides known as booster biocides. Trials of alternative copper-based coatings with rapidly degradable boosting biocides on ships in Japan have claimed recent breakthroughs with equivalent performance of TBT products (ENDS Report 1998).

Although at present copper antifouling paints present the BPEO available to the marine industry, there are a number of potential environmental impacts that may occur from using copper antifouling paints. Copper present in the water and sediments can be accumulated by benthic animals causing, for example, reduced respiration rates and impaired growth in mussels, clams and other shellfish (Sobral & Widdows 1997). The toxicity and accumulation of copper varies greatly depending on concentration levels, exposure, temperature and salinity, the presence of other metals and the type, size and age of the marine organism. It is therefore difficult to generalise about the toxicity of copper to marine organisms, there is evidence that certain species of fish are sensitive to quite low levels of copper even though other species are tolerant of much higher levels. Benthic marine organisms are thought to be slightly more sensitive to copper than fish, although some species demonstrate a capacity to adapt to elevated levels.

There is limited information available on the environmental impacts on non-target species, particularly algae, associated with the use of the newer booster biocides, such as the herbicides irgarol and diuron. These studies are discussed in the Recreational User Interaction report (UK CEED 1998 and 1999 in preparation 1999). Using a model to predict concentrations of antifouling in the environment, WRc have estimated that the six most common biocides used in antifouling paint for recreational craft, including copper (1) oxide, diuron, copper thiocyanate and ‘Irgarol 1051’, were present in marina waters in concentrations generally more than an order of magnitude higher than levels required for toxic effects on marine algae and fish (Boxall, Conrad & Reed 1998). However, it should be noted that these estimated concentrations were generally higher than levels actually measured in the marine environment and are likely to be an overestimate. An improved model is currently being developed for the HSE and the Environment Agency, which should provide information that will help to determine whether further control options are necessary.

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