Underwater Antifouling Coatings

Effective underwater hull coatings determine performance factors including, speed, fuel consumption and weight of a vessel. Environmental regulations and performance life have been a major concern for underwater hull coatings known as antifouling coatings. These coatings must comply with several environmental regulations. Among these regulations are the air emissions standards, VOCs regulations, Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) and NESHAP regulations. The EPA must also register the coatings. Antifouling coatings should have the ability to be applied to a substrate using standard protective equipment. Finally, they must also be NMCPHC approved. The goal of the antifouling coating is reduce the amount of marine growth on the coating while maintaining compliance with environmental regulations.

The underwater hull of a ship is subjected to not only the corrosive environments of the ocean seawater, but to the constant accumulation of biofouling. Biofouling or marine growth includes any seawater microorganisms such as tubeworms, mussels, barnacles and algae. Marine growth is a major problem to encounter when dealing with the underwater hull. The microorganisms cause inconsistencies in the coating surface and create friction with the water. In turn, this friction decreases the speed of the vessel and adds weight to the hull. Both of these factors increase fuel consumption and inflate the cost of maintaining the vessel. Biofouling is also a costly process to prevent, remove and repair. There are processes available to remove the fouling once it has formed. These processes are effective but the coating surface has been agitated by the initial growth and then more susceptible to further marine growth. The ideal antifouling coating would prevent marine growth as well as maintain a long performance life while keeping within strict environmental regulations.

There are two main types of underwater hull antifouling coatings. Chemical release coatings use biocides, or chemical toxins that are released into the seawater and prevent marine organisms from attaching to the surface. The toxins create a barrier that prevents the marine growth. In the past these coatings were typically either organotin or copper oxide. Some of the chemicals that give these products their toxic properties include; cuprous oxide, mercury, copper, arsenic and tributyltin oxide (TBT). Any combination of these chemicals provides a harmful biocide to the aquatic environment.

There are two methods of biocide release for these chemical antifouling coatings. The first method uses a binder within the coating matrix that breaks down in the seawater and slowly releases the toxins into the surrounding area. The second method uses a leaching method to release toxins. As one toxin particle is released, another particle is right behind it ready to repeat the process. Chemical antifoulants have a service life of 18 months to as much as 8 years, requiring replacement within a few years of application.

Another type of underwater hull coating is an ablative, self-polishing coating system. Ablative systems prevent marine sea life from attaching sufficiently to the coating surface. The initial coating surface steadily dissolves in the seawater. As the top layer dissolves, a new smooth layer is left behind to repeat the process. The rate of replenishment is controlled and constant allowing a uniform transition through each layer of the coating. This rate at which these coatings dissolve is accomplished by a chemical reaction between the seawater and a chemical binder contained within the coating matrix.

The first ablative antifoulant was produced with the biocide organotin or tributyltin. The biocides are chemically linked to a polymer base. The polymer links are steadily broken and the top surface is released into the seawater during hydrolysis. A smooth fresh surface of biocide is left behind to begin the process again. These coatings are intended to last five years.

TBT and many other biocides have caused serious environmental problems within inter tidal zones and ocean sediments. These non-target areas are primary concern in protecting against the hazardous chemicals used in the composition of antifouling coatings. Many of these chemicals have been put under strict regulation or even banned by organizations such as the International Maritime Organization (IMO) (See TBT Ban). The restrictions are expected to become more stringent over the next few years as environmental concerns tighten. These restrictions have caused a movement towards new biocide developments or methods that are not as harmful to the environment. Antifoulants must also abide by other regulatory laws including VOC air quality standards, Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) and HAP air pollutant laws. This provides a very tight performance specification for the chemistry of these coatings.

The tin-based systems were replaced by tin-free ablative copper oxide. Though the organotin systems were found to be more effective, the copper biocide is a more environmentally friendly coating. Composition of these ablative antifoulants consists of the biocide cuprous oxide with rosin in a polymer base or acrylic resin. This composition works in the same manner as the previous tin-based system. The expected performance rate is up to 8 years and is variable depending on the coating thickness.

Another type of ablative antifoulant is called easy-release or foul-release. Easy-release coatings use a silicone base resin with the concept that a slippery surface does not provide an adequate attachment for marine growth. When a vessel is at rest the marine life may settle onto the coating but as a vessel moves through the water, the marine growth simply slides off. If growth does successfully adhere, it is easily removable and less costly than other antifoulants due to its slippery surface. They are subject to abrasion and are sensitive to the cracking and peeling. Easy-release coatings are non-toxic, contain no biocides, and are more environmentally friendly. The easy-release coatings last up to four years, however, are more expensive due to the chemistry and greater amount of application layers required.

Another development is the co-biocide or booster biocide. The co-biocides will breakdown better in a marine environment providing a less harmful mechanism against growth and non-target organisms.

More recently copper use as a biocide has been under scrutiny. Tighter restrictions are causing more and more studies to be directed towards the development of a low-toxic or non-toxic reliable antifouling coating. A better quality, environmental compliant antifouling coating is in high demand. Antifoulants that have all of the important performance qualities would reduce the budget cost of managing underwater hull coatings.

TBT Ban
There are many environmental regulations that are protecting the current use of anti-fouling coatings and the chemical content of the coating, including Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), NESHAP regulations, VOC emission regulations, and the clean air act (CAA). The latest regulation comes from the Environmental Protection Agency (EPA) and International Maritime Organization (IMO) in conjunction with 25 countries to create the Anti-Fouling Treaty.

The Anti-Fouling Treaty places a ban on TBT or organotin biocides used in underwater anti-fouling paints. The treaty was finalized in October of 2001 and is still being reviewed and implemented. The treaty has not been registered yet but has already set the dates for the adoption of the organotin ban. Currently, the IMO is asking for a voluntary cut-off of the use of organotins due to the impending treaty registration. The treaty specifies the start of the ban to begin on January 1, 2003. This ban will prohibit the application or re-application of TBT coatings to underwater structures. The treaty also specifies January 1, 2008 for the complete ban of TBT coatings, requiring that structures must have all the coating removed or overlay a protective barrier to prevent the leakage of organotin biocides. This choice will be left to ship owners for choice of removal or overcoat. The cost of over-coating TBT is much cheaper than that of removal, however, inspection of over-coated organotins will be required. The barrier coat would be required to prevent any delaminating and leak through. New tests for TBT coatings are being developed and reviewed. Some of these tests include lab analysis, x-ray fluorescence and ion scanning. Ship owners that decide to remove the coating completely will have to adhere to strict removal regulations. The removed waste stream must be caught and removed of entirely.

It is predicted that treaty will not be completely registered until after the ban date of January 2003, however, the dates are fixed. The set dates will not be changed if a late release date of the treaty occurs. Therefore, if the treaty is not approved and implicated until 2005 the requirement of no application of TBT coatings will be immediately implemented.

The treaty leaves an open opportunity for future biocide restrictions and bans. One of the predicted bans would limit the amount of copper (Cu) emissions. Each country has different regulations placed on their anti-fouling coatings. Canada, for example, already limits the amount of copper emissions. The Netherlands prohibits the cleaning of copper based anti-fouling coatings. The Anti-Fouling treaty is working towards reciprocal banning. This form of banning would require that two countries agree to ban the same coatings according to a bilateral agreement between the two coating regulations. Both countries would require the same strictness in testing, reviews and approval. The treaty will not have a "approved products list" of anti-fouling coatings. For more information including treaty drafts and information links, please see www.antifoulingpaint.com.