Smooth Sailing a history of ZPT
In the 1970s, tributyltin (TBT) compounds, which were effective against both soft (e.g., algae), and hard fouling organisms, were introduced. For antifouling agents like copper and tin compounds to be effective, they must be continuously released from a hull’s surface at some minimum rate. The toxicity of released copper appears to be rapidly neutralized in an aquatic environment by complexation, adsorption and other naturally occurring processes (Hall, 1999). However, released TBT is persistent in the environment and has adversely affected populations of non-target organisms.
The negative impact of TBT has led to usage restrictions by many governments and to an intense search for alternatives. At the International Maritime Organization (IMO) Marine Environmental Protection Committee meeting (MEPC-42), a resolution was drafted calling for the ban on the application of all antifouling paint containing organotins used as biocides, effective Jan. 1, 2003. It also calls for the elimination of the presence of these coatings on all vessels effective Jan. 1, 2008. The IMO is expected to adopt this resolution in November of this year. This increases the urgency to identify materials that will disrupt the lifecycle of fouling organisms, and thus prevent their accumulation and growth on the surfaces they protect, while not impacting the environment.
One approach is using a co-biocide (algaecide), which is effective against soft fouling organisms, in combination with cuprous oxide, which is effective against hard fouling organisms. The ideal co-biocide must have broad spectrum activity since many organisms contribute to fouling, low water solubility for performance longevity, a history of safe usage, and environmentally acceptable properties, i.e., not adversely affect non-target populations. Since the environmental risk is directly related to the environmental concentration, the successful antifoulant must be non-persistent when released into the aquatic environment. Zinc OMADINE®* fungicide-algaecide (see Figure 1) or zinc pyrithione (ZPT) and copper pyrithione (CuPT) are two of several pyrithione compounds that have these properties.
Effectiveness of OMADINE Fungicides-AlgaecidesZinc or copper OMADINE biocide in combination with cuprous oxide or copper thiocyanate provides very effective control of both hard and soft fouling. Immersion testing has been carried out at Miami Marine with paints containing ZPT as a co-biocide. Panels were coated with paints containing the following.
- Gel-coat only (no biocide),
- 56% copper oxide (Cu2O)/wood rosin,
- 55% Cu2O plus 5% ZPT/glycerol rosin, and
- 55% Cu2O plus 5% ZPT/diethylene glycol rosin.
The panels were immersed for 20 months. Figures 2 and 3 show that while copper oxide alone inhibited fouling compared to the control panel without biocide, the combination of Cu2O and ZPT showed considerably less fouling. The beneficial effect of copper thiocyanate and ZPT is even more dramatic (see Figure 4).
After 8 months’ immersion, there is significant fouling with copper thiocyanate alone, whereas with the combination of copper thiocyanate and ZPT, there is very little fouling. Figure 5 shows a naval patrol boat stationed in Cairns, Australia, with test patches of different antifouling paints after 15 months’ service. Patch 2, which shows the best efficacy, is a copper acrylate system containing zinc OMADINE. This paint continued to show good performance until the test was terminated at 4 1/2 years.
Efficacy of Copper Omadine Biocide in Antifouling PaintUsing a published U.S. Navy antifouling paint formulation (organic solvent carrier, vinyl resin) as a guide, a series of prototype paints containing combinations of CuPT with Cu2O were prepared. A statistical experimental design (ECHIP™) was used to study different levels of antifoulants. Painted panels were immersed in the ocean at Miami Marine Research and Testing Laboratory (Biscayne Bay, FL) and evaluated monthly for resistance to fouling. Controls included a biocide-free blank formulation, an antifouling paint based on TBT with Cu2O, and a commercial antifouling paint based on Cu2O with a soluble copolymer matrix. A sample formulation is given in Table 1. Exposure results obtained after 10 months of total immersion are shown in Table 2.
Multiple regression analysis of the data yielded a model that explained 90% to 95% of the experimental variability (r2 = 0.90 to 0.95). Key findings were:
- combinations of CuPT and cuprous oxide are more effective than either biocide alone,
- relatively low levels of CuPT (~ 3%) are effective,
- and CuPT/cuprous oxide paints perform better than commercial controls.
CuPT and cuprous oxide appear to act in concert to provide total antifouling performance. For this type of formulation and under these testing conditions, the model predicts an optimal formulation of 3.2% CuPT and 40% Cu2O. These conclusions are supported both by visual examination of test panels as well as by the strong CuPT-cuprous oxide interaction term in the statistical model.
Assessing the Environmental Risk of OMADINE Antifouling BiocidesSeveral studies of zinc and copper pyrithione were conducted to assess environmental fate, persistence and impact. These studies include leach rates, abiotic hydrolysis and photolysis, aerobic and anaerobic aquatic metabolism, adsorption/desorption, and die-away studies. Most studies followed U.S. EPA Pesticides Assessment Guidelines; the leach rate study followed the ASTM D 5108-90 protocol established for tributyltin paint.
Leach RatesThe main route of entry for antifouling agents into the aquatic environment is by leaching from painted hulls. The leach rates of zinc and copper pyrithione have been determined for several ablative, U.S. Navy 121 and self-polishing coatings. Figure 6 shows the leach rate profile of Zinc OMADINE biocide from the U.S. Navy 121 formulation; the steady state, 21 to 45 day leach rate was 2.3 (µg/cm2/day). CuPT has a much lower leach rate in the same formulation reflecting its lower water solubility.
Hydrolysis and PhotolysisZPT and CuPT hydrolysis and photolysis were studied in sterile synthetic seawater and fresh water. Whereas the half life for hydrolysis of pyrithiones in dark sterile waters ranged from 7 to more than 90 days, the photolysis half life ranged from 15 to 30 min. Environmental photolysis rates for compounds with high quantum yields, e.g., the pyrithiones, depend upon various factors including angle and intensity of incident surface solar radiation, water depth and clarity, presence of photosensitizers and particulate matter, and wave action. It is clear, however, that radiation in the 300 to 355 nm range, which causes photodegradation of pyrithione, can penetrate to significant depths under favorable conditions (Smith, 1976). Therefore, it is likely that photochemical processes play an important role in the environmental degradation of pyrithiones.
Aquatic MetabolismBiotic degradation of ZPT and CuPT were studied under dark, aerobic and anaerobic conditions. Anaerobic degradation was very rapid. The first and second half-lives (75% disappearance) appear to occur in less than one hour, while sorbed pyrithione shows a slower, although still rapid, degradation. Aerobic aquatic metabolism shows a similar bimodal degradation. The first half-life occurs very rapidly with subsequent half-lives following a slower kinetic process. The aquatic fate of ZPT and CuPT are summarized in Figure 7.
Predicted Environmental Concentration (PEC)Data from these studies can be used to predict environmental concentration, fate and persistence of zinc and copper pyrithione in harbors and lakes. Using U.S. EPA software (EXAMS2.97, Burns, 1996), the PECs in the water column and in the sediment were calculated for different environments, including a worst case harbor, which was simulated by using actual measured concentrations of persistent antifoulants (Scarlett, 1997; Rexrode et al., 1997). In a harbor where a persistent antifoulant would have a PEC of 400 ppt, the pyrithiones are calculated to have a PEC of ~15 ppt in the water column and less than 1 ppt in the sediment. EXAMS predicts that the major degradation pathways for pyrithione are photolysis and biodegradation. These predictions are supported by recent harbor monitoring studies carried out in southern England where pyrithione could not be detected (Thomas, 1999).
Risk AssessmentsThe major question in assessing the risk of using a biocide in an application is: Will the environmental concentration reach the toxicity threshold for non-target organisms? The LC50s of pyrithiones range from 2.6 to 400 (g/L for aquatic organisms. A worst case PEC for the pyrithiones is ~0.015 (g/L. An indicator of the risk is the risk quotient: RQ = PEC/ LC50 = 0.015/2.6 = 0.006. A biocide application associated with RQ less than 1 is considered a low risk. Even if the RQ is multiplied by an uncertainty factor of 100, the RQ remains less than 1. These calculations are consistent with the long history of pyrithione usage and the absence of ecological effects.
SummaryZinc or copper OMADINE biocides, together with copper oxide, are highly effective antifoulants in marine paints, protecting against both soft and hard fouling organisms. Over 3,000 vessels, painted with commercially available antifouling paints, have shown excellent antifouling performance for as long as 4 1/2 years. Studies to determine the environmental fate and effect of pyrithione show that it is non-persistent. The major routes of degradation are photolysis, aquatic metabolism and sediment-catalyzed decomposition. Modeling predicts that the steady state environmental concentration for zinc or copper pyrithione would remain significantly below the toxicity threshold for a variety of aquatic organisms, including those with the lowest no effect concentration values. Zinc and copper OMADINE biocides are two materials that give good antifouling performance without negatively impacting the environment and, therefore, provide excellent alternatives to organotins used as biocides in antifouling paints.
AcknowledgementsThe authors wish to thank Dr. Robert Fenn, Mr. James Ritter, Mr. Christopher Bannon and Dr. Nicholas Skoulis for their many technical contributions to understanding the environmental fate and toxicology of the metal pyrithiones.
For more information on biocides, contact Arch Chemicals Inc. at 350 Knotter Dr., Cheshire, CT O6410; phone 203/271.4000; fax 203/271.4050; visit www.archchemicals.com.