|Antifouling Coatings Standards Keep Pace with Technology Changes
by Elizabeth Haslbeck
The deleterious effects of the growth of marine fouling organisms on ocean going vessels have been well known since man first put to sea. Fouling organisms such as barnacles, tube worms and algae accumulate on any submerged surface, greatly increasing hull drag and reducing the speed and fuel economy of vessels. There is an enormous cost associated with controlling fouling, and many efforts have been directed toward both understanding the fouling phenomenon and the development of effective marine hull-coating systems to combat the problem.
Antifouling coating technology has evolved from simple resin-rosin systems (1950s-1960s) based on the biocide cuprous oxide (Cu2O), to copper ablative coating systems (1970s and 1980s). At about the same time, organotin-based self-polishing co-polymer coatings were introduced. At present, tin-free self-polishing systems that contain organic booster biocides in addition to cuprous oxide are being introduced into the marketplace. Another recent development includes biocide-free, foul-release materials that take advantage of low coating surface energy to minimize the ability of fouling organisms to adhere tightly to the coated surface.
The ability to demonstrate efficacy of any fouling control system is important. As coating technology has evolved over time, the series of standardized tests to facilitate this process has also evolved. No longer are methods limited to the assessment of fouling accumulation in field/panel testing. ASTM Subcommitteee D01.45 on Marine Coatings currently supports standards in three basic categories biocide release, fouling control, and biofouling adhesion.
Biocide Release Rate
Over the years, many biocides have been used in marine antifouling coating systems, the two most frequently used being copper in the form of cuprous oxide and tributyltin (TBT). In the 1970s and 1980s, negative effects of TBT on non-target organisms were identified, and an ASTM TBT release rate measurement method was developed (ASTM D 5108) and used to establish limits on release rate. Through the establishment of maximum allowable TBT release rates, the use of coating systems with excessively high release rates was prohibited.
Currently, TBT-based coatings are being phased out and will likely be banned when the International Maritime Organizations tin coating prohibition treaty is ratified by the required number of member states. As was shown with TBT, the critical issue for any biocide with regard to its environmental impact to non-target organisms is its site-specific loading rate, dispersion, and persistence in the environment, which are directly related to the release rate of the biocide from the hull under natural conditions.
The rate of biocide release from antifouling (AF) coatings used to control marine fouling has been studied for decades, and has been found to be influenced by both physical and biological factors. Hydrodynamics, temperature, pH, salinity, and the presence of biofilms (communities of bacteria and algae) on the coating surface dramatically affect release rate. (1,2) Additionally, basic coating chemistry heavily influences release rate. This has been observed over a three- to four-decade period starting in the 60s and 70s with free association coatings where copper in the form of cuprous oxide was freely associated into a vinyl resin-rosin paint matrix. The next generation, ablative copper paints, were developed in the 70s and 80s. The primary release rate moderator in these coatings was more of an erosion control phenomenon. Now, in the 90s and early 2000s, emerging coating systems are based on self-polishing resins that control release rate through a combination of hydrolysis/dissolution in seawater.
Experimental approaches to measuring biocide (TBT and copper) release rates have evolved over time. With ASTM developing the rotating cylinder release rate methods (D 5108, Test Method for Organotin Release Rates of Antifouling Coating Systems in Sea Water, and D 6442, Test Method for Copper Release Rates of Antifouling Coating Systems in Seawater), rotating cylinder methods are emerging as the accepted methods for measuring TBT and copper release rate from antifouling coatings in the laboratory. ASTM D 5108 was originally developed to provide a mechanism by which TBT-based coatings could be compared one to another, and ultimately was useful in restricting use of tin-based coatings with extraordinarily high release rates. Methods are now under development for the measurement of release rates of organic biocides from advanced antifouling coating systems.
Interestingly, the rotating cylinder methods were not intended to estimate in-service rates of release of biocides, and thus the data are of limited use for environmental fate and effects studies. To support fate and effects work, a suite of release rate methodologies has evolved, including combinations of field/lab release protocols that combine field aging with laboratory release measurements. In addition, an in-situ dome method was developed by researchers at the Space and Naval Warfare Systems Center, San Diego (3) to measure biocide release rates directly from vessel hulls. With respect to TBT, the release rates measured using the ASTM rotating cylinder method were five to 39 times higher than those measured by the in-situ dome method developed by the U.S. Navy. (4) Others have reported this tendency toward higher ASTM values as well. (5) The relationship between results from the array of methods known today remains undefined. The development of new methods to provide an improved understanding of release rate, and/or the adaptation of models or other predictive tools may be useful in that regard.
Numerous ASTM tests have been developed to support the evaluation of coating performance in the field. The most basic test method is ASTM D 3623, Method for Testing Antifouling Panels in Shallow Submergence, and involves exposing coated panels at constant depth (also known as shallow submergence testing). In this test, panels are held vertically in exposure racks suspended from a floating raft, and are periodically inspected for accumulation of fouling organisms and for physical performance.
Because the overwhelming majority of vessels experience periods of both static and dynamic activity, constant depth static exposure testing may not be sufficient to fully assess coating performance. Thus ASTM developed ASTM D 4939, Test Method for Subjecting Marine Antifouling Coating to Biofouling and Fluid Shear Forces in Natural Seawater, which combines static and dynamic exposure periods. Coated curved panels are attached to the outer surface of a rotating drum, and during a dynamic cycle the drum is rotated so as to produce an apparent velocity to the coating of approximately 15 knots. Panels may be evaluated for fouling accumulation and coating physical performance including, but not limited to, loss of coating thickness due to hydrodynamic aging.
The waterline area of a vessel is particularly challenging to antifouling coating performance. At the waterline the coating is subject to wave action, abrasion from floating debris, oils and contaminants that partition to the waters surface, and direct sunlight. Thus, ASTM developed ASTM D 5479, Practice for Testing Biofouling Resistance of Marine Coatings Partially Immersed, to address this type of environment. For this test, 15 x 46-cm panels, coated with the antifouling system, are exposed from a floating raft, and held vertically in place parallel to tidal currents. The top 10 cm of the panel remain above the water surface. Panels are then evaluated in accordance with D 3623 with special notes made on the performance of the material at the waterline.
Low surface energy coating systems (foul-release coatings) have been developed for biocide-free fouling control. These materials take advantage of specific surface properties to minimize the adhesion strength between the fouling organism and the coating surface. Therefore, although the coating system does not prevent fouling from settling and growing on a surface, the fouling is easily removed usually under hydrodynamic flow. Traditional fouling rating protocols are useful for tracking the accumulation of fouling on such coating systems, but they tell the inspector very little about the ease with which the fouling can be removed. Scientists at the Florida Institute of Technology developed a technique whereby a spring gauge (force gauge) was employed in shear to push a fouling organism off of a fouling-release surface. Knowing the surface area of the organism and the force required for organism removal, a removal stress can be calculated. ASTM has developed and published this method (D 5618, Test Method for Measurement of Barnacle Adhesion Strength in Shear), and it has been found particularly useful in predicting which fouling-release coating systems will self-clean under hydrodynamic flow.
Development of antifouling coatings is currently in a state of flux. New resin systems are being introduced, along with myriad alternative biocides, and the regulatory environment is constantly changing. In light of this, the role of Subcommitteee D01.45 in bringing forward timely and high quality consensus standards becomes even more important. ASTM looks for active participation from researchers, end-users, and paint companies to meet the global need for standardization through the development of methods, techniques, and protocols used to assess coating performance. //
The author would like to acknowledge the Office of Naval Research, which has contributed funding to universities and independent laboratories in the area of coating research. The funding did not directly support ASTM, ASTM subcommittees, participation of ASTM subcommittee members in ASTM activities, or the development of specific standards, but indirectly contributed to the development of knowledge in the areas of foul-release and antifouling coatings.
(1)Woods Hole Oceanographic Institution (1952) Marine Fouling and its Prevention. United States Naval Institute, Annapolis
(2)Mihm, J.W. and G.I. Loeb. (1988) The effect of microbial biofilms on organotin release by an antifouling paint. In Biodeterioration 7 (pp. 309-314). D.R. Houghton, R.N. Smith and H.O.W. Eggins (Eds.). Elsevier Applied Science, London.
(3) Haslbeck, E., A. Valkirs, P. Seligman, A. Zirino, Ignacio Rivera, J. Caso, and E. Chen. (2000) Release rate determination and interpretation for copper antifouling coatings. In Assessing the Future of Coating Work. The Proceedings of the PCE 2000 Conference and Exhibition (pp. 329-348). Genoa, Italy 8-10 March 2000.
(4)Schatzberg, P. (1996) Measurement and Significance of the Release Rate for Tributyltin. Chapter 19, pp. 383-403 In Organotin: Environmental Fate and Effects. M.A. Champ and P.F. Seligman Eds. Chapman and Hall. New York. 623 pp.
(5)Thomas, K., K. Raymond, J. Chadwick, and M. Waldock. (1999) The effects of short-term changes in environmental parameters on the release of biocides from antifouling coatings: Cuprous oxide and tributyltin. Applied Organometallic Chemistry, vol 13, pp. 453-460.
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