Anti Fouling Paints

TBT Concerns

Based on the positive results seen from the ban of tributylin (TBT) anti-fouling paints on aluminum pleasure craft throughout Europe and the U.S., several Delegations have intensified their efforts to accelerate the implementation of regulation which would ban the use of TBT anti-fouling paints for all commercial ships. A primary concern to the shipping industry is the performance characteristics of TBT alternatives.  

TBT Alternatives

The most promising alternative appears to be copper acrylate self polishing copolymer (SPC) paint which leaches into the water column organic copper biocides in a much more controlled and continuos manner than the release of butylin residues from TBT paint. The leaching process occurs either by hydrolysis (where the surface layer of the paint film reacts chemically with water leaving a relatively polished surface) or by hydrophylic self depletion (the surface layer is physically removed by the action of the water leaving a fresh surface). The latter process results in a slightly rougher surface due to the partial accumulation of depleted layers which do not completely loosen and dislodge. Although international regulations and the requirements of IACS Members require dry docking at 5 year intervals, it has been reported that approximately 95% of the ships (expressed in terms of deadweight) dry dock on a 2-1/2 year cycle and therefore the performance of such alternatives need only be evaluated for this duration.

According to ABS’ records, for ships greater than 500gt, only 75% of the ships (expressed in numbers of ships) dry dock at this reduced interval. Both sets of statistics indicate that a clear majority of ships dry dock at a period less than the 5 year maximum.

Phase Out of TBT

TBT risks to aquatic life is known to be very serious. What remains uncertain, although preliminary analytical models have predicted negligible effects, is the level of copper concentrations that would result if copper SPC’s were to be extensively used as a result of a total ban of TBT paints. Another uncertainty is the maximum level of toxicity considered to be safe. MEPC decided that the phase out of TBT should be considered at its next session in November with the view that a new annex to MARPOL should be developed.

The exact time frame for a total ban of TBT paints was discussed and there was general consensus that a global phase out should be implemented within the next 5 to 10 years. This may be accepted depending on the evaluation of several issues including the efficacy of TBT alternatives relative to the 5 year dry docking period vs the shorter periods used by industry.