ANTI-FOULING PAINT

MARINESHELF publishes articles contributed by seafarers and other marine related sites solely for the benefit of seafarers .All copyright materials are owned by its respective authors or publishers.

 Anti-fouling paint or bottom paint is a specialized coating applied to the hull of a ship or boat to slow the growth of subaquatic organisms that attach to the hull and can affect a vessel's performance and durability. Hull coatings may have other functions in addition to their antifouling properties, such as acting as a barrier against corrosion on metal hulls, or improving the flow of water past the hull of a fishing vessel or high-performance racing yacht.

Tributyltin (TBT) is an umbrella term for a class of organotin compounds which contain the (C₄H₉)₃Sn group, with a prominent example being tributyltin oxide. For 40 years TBT was used as a biocide in anti-fouling paint, commonly known as bottom paint, which was applied to the hulls of ocean going vessles. Bottom paint improves ship performance and durability as it reduces the rate of biofouling, which is the growth of organisms on the ship's hull. Although such paints are effective, the TBT slowly leaches out into the marine environment where it is highly toxic to a wide range of organisms. TBT pollution is of serious concern as it has led to collapse of whole populations of organisms.
TBT compounds are organotin compounds, with 3 butyl groups covalently bonded to a tin(IV) centre. A general formula for these compounds is (n-C₄H₉)₃Sn-X. The X group is typically an electronegative "leaving group" such as chloride or carboxylate.. When introduced into a marine or aquatic environment, TBT adheres to bed sediments because of its high specific gravity and low solubility. However, the adsorption of TBT to sediments is reversible and depends on pH. Studies have shown that 95% of TBT can be released from the sediments back into the aquatic environment. This release makes it difficult to quantify the amount of TBT in an environment, since its concentration in the water is not representative of its availability.


Because TBT is the most effective anti-fouling agent discovered, it was frequently used in anti-fouling paint throughout the globe. It is also relatively inexpensive.
The antifouling properties of TBT compounds were discovered in the 1950s in the Netherlands by van der Kerk and coworkers. The function of the biocide in the anti-fouling paint is to prevent the settling of organisms on the hull and to poison the organisms that do. Although an effective biocide, tributyltin was wrongly deemed safe environmentally. By the mid 1960s it became the most popular anti-fouling paint worldwide. TBT was mixed into paints to extend the life of antifouling coatings, and ships were able to continue operations for a longer time frame. The paints ensured fuel efficiency and delayed costly ship repairs. It is also an ingredient in some disinfectants, for example in combination with quaternary ammonium compounds.








TBT compounds are banned and are included in the Rotterdam Convention and have been banned by the International Convention on the Control of Harmful Anti-fouling Systems on Ships of the International Maritime Organization.
Bans on TBT on boats less than 25 metres long first started in the 1980s. In 1990, the Marine Environment Protection Committee adopted Resolution MEPC 46(30), which recommended that the Government eliminate the use of TBT-containing antifouling paints on smaller vessels. This resolution was intended to be a temporary restriction until the International Maritime Organization could implement a complete ban of TBT anti-fouling agents for ships. Several countries followed with a ban of use, and in 1997 Japan banned the production of TBT-based anti-fouling paints.
The use of organotin compounds acting as biocide in anti-fouling paint was completely banned in 2008 by the International Convention on the Control of Harmful Anti-fouling Systems on Ships of the International Maritime Organization. It states that ships cannot bear organotin compounds on their hulls or external parts or surfaces unless there is a coating that forms a barrier so that organotin compounds cannot leach out. This measure helps reduce exposure by allowing recovery to occur. Despite the ban, TBT will most likely be present in the water column and sediment for up to twenty years because of its long half-life.

Violations of the ban on TBT

Even though banned by some international agencies, TBT anti-fouling paints are still being used in countries with poor regulation enforcement, such as countries in the Caribbean. TBT can remain in the ecosystem for up to 30 years.

Antifouling Convention prohibits the use of underwater antifouling paints containing TBT

On 17 September 2008 the International Convention on the Control of Harmful Antifouling Systems on Ships, 2001 has entered into force globally. This mandatory convention prohibits the use of antifouling paints containing TBT on ships.

All ships of 400 GT and above in international trade must have a TBT free antifouling coating on their hull or must have applied an approved sealer coat. Further details of the hull coating are listed in the supplement of the International AFS certificate or in the AFS declaration.

Ships under 400 GT need only an AFS declaration which can be issued by the ship owner himself based on information provided by the paint manufacturer.

Ban of underwater antifouling paints containing TBT applies in domestic trade, too

The use of underwater antifouling paints containing TBT on ships of 400 GT and above is also prohibited by European law. Other than the International Convention on the Control of Harmful Antifouling Systems on Ships the European regulation (EC) No 782/2003 is also applicable for ships engaged in national trade.