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Undersøgelse af kritisk frigivelseshastighed for kobber fra bundmaling til lystbåde 

Summary and conclusions

Background

Copper is used as active substance in antifouling paints applied on ships' hulls to prevent the settlement of marine organisms on the surface, and also to protect wooden ships from being invaded by shipworms. Copper, when released to the aquatic environment, is normally complexed with organic or particulate material. Sedimentation removes the copper from the water body to the sediment, where it is tightly bound, and therefore no longer bioavailable. However, in Denmark concerns have been raised over the use of copper in antifouling paints, due to elevated concentrations of copper in the harbour sediment, which complicates regulations regarding dredging of harbour sediment.

The purpose of this project is to find the lowest possible release of copper from pleasure craft antifouling paints that at the same time exhibits a satisfactory antifouling effect

Test paints

Based on tests with 10 antifouling paints with different properties and copper release rates during the sailing season 1999, two test paints were selected for practical testing in the 2000 season on a total of 59 yachts in 7 different Danish marinas.

At the beginning of the season, the two paints, light blue and dark blue respectively, were applied on the port and starboard sides of the 59 test boats (the colours chosen represent a different release rate of copper).

Three types of results were collected in the project using questionnaires, diving inspections and laboratory measurements.

Questionnaires

At the start of the sailing season, following the application of the paints, the boat owners completed a questionnaire about the application properties of the paints and about their expectations concerning the season's sailing activities. At the end of the sailing season (before bringing the boats on land), the boat owners once again completed a questionnaire. This questionnaire asked about sailing experiences related to the 2 paint types, fouling intensity, actual sailing activity during the season and general impression of the paints compared to each other and to previously used antifouling paints.

In general the boat owners were very satisfied working with and using both paints and they did not experience that the hull had been applied two different systems. Only little fouling was observed in the water line on either side of the boats during the season, although the dark blue was noted to foul a little more than the light blue.

Diving inspections

A diver inspected all participating yachts three times during the season (week 25, 34 and 40). During the first inspection, more slime was observed on the dark blue than on the light blue paint, and at the second and particularly the third inspection the difference was remarkable. On the light blue, only slime was observed all through the season. On the dark blue a thick layer of slime was observed, and growth of brown algae was beginning to appear in a few cases.

Laboratory measurements

The release rate of copper from the two paints has been measured according to ISO 15181 at DMU (National Environmental Research Institute of Denmark) in Roskilde, Denmark. The light blue paint has a normal copper release profile (compared with available commercial antifouling paints) with a high initial release, which after approx 20 days decreases to a constant level. The dark blue paint, on the other hand, has a very low initial release rate which after a short period increases to a level at least 20% below the release rate level for the light blue product relatively.

Comparing the accumulated amount of copper released from the two paints, it appears that during the first 14 days, dark blue releases approx 80% less copper in total than light blue paint. During the first month, the release of copper from dark blue paint is more than 50% below the release level from light blue.

Similar paints have been tested according to ASTM D5108-90 at Thalassa in Uppsala, Sweden. Similar results were found as regards the ratio between the release rate and the total amount of copper released from the two paints.

Unfortunately variations in the results obtained using these methods of leach rate measurements are at present still high. At DMU, the ISO 15181 method has shown variations of up to a factor 2 in the copper release rate from one test to another. At present these methods are therefore not considered appropriate for establishing release rate limits (limit values) of copper from antifouling paints.

However, the methods can be used to determine the copper release level relatively between two or more paints in the same test (variations under 15%).

Conclusion

The dark blue (low release rate) paint is less successful in preventing slime build up on the hull of the boats, than the light blue (standard release rate) paint. The dark blue paint is covered with a thick layer of slime, and in a few cases at the end of the season additionally e.g. some brown algae and a few barnacles. However, use of the dark blue paint will result in an environmental bonus due to an expected reduced level of copper release of more than 50% during the first month after launching. Typically this is the time of the season when relatively intense fouling appears.

Based on this, Dansk Sejlunion (Danish Sailing Association) and presumably the majority of the pleasure craft owners in Denmark would accept paint such as the dark blue (low release rate). But it should be regarded as the absolute lower limit concerning the antifouling properties required by an antifouling paint.

Because of the large variations in the results of obtained by the ISO method it does not seem scientifically appropriate to establish an actual copper release rate limit for an antifouling paint which at the same time will provide an acceptable level of antifouling effect. Based on this it is recommended to use the dark blue paint as reference paint in case restrictions on the release of copper from antifouling paints into the aquatic environment are introduced in Denmark.