Marine biofoulants that attach themselves to and grow on a ship's hull increase drag as a vessel moves through water. This drag leads to increased fuel consumption, and it can limit the maximum attainable speed, thus impairing a ship's operational capability. To prevent this buildup, antifouling coatings are applied to the hulls of ships. Unfortunately, these coatings contain toxic metals such as copper that leach into the water and accumulate in harbors. Several nations plan to ban ships coated with these metal-containing paints from their harbors. Using current antifouling technology, Department of Defense (DoD) as well as commercial vessels will be denied access to critical harbors around the world. New environmentally friendly antifouling technologies are needed to replace the current, heavy metal-containing coatings.
The objective of this project was to develop non-toxic, copper-free, environmentally benign, antifouling polymer coatings of controlled modulus and surface properties. A secondary objective was to determine whether low surface energy fluorinated coatings or hydrophilic polyethylene, glycol-based coatings provided the best fouling release behavior.
The antifouling coatings developed by this project consist of two layers, an elastomer base layer and a surface-active block copolymer (SABC) fouling release layer. A styrene-ethylene/butylene-styrene (SEBS) thermoplastic elastomer (TPE) was used as the base layer for these coatings as it provides corrosion protection, adhesion, durability, and is commercially available and relatively inexpensive. The antifouling properties of the coating were provided by the thin SABC layer. Preliminary studies showed that both non-polar fluorinated and polar polyethylene, glycol-based SABCs showed promise as antifouling coatings. This project evaluated the molecular-level characteristics, performance, and fouling release behavior of these materials to determine which is superior.
To determine the optimal surface energy required for foul release applications polymers with different ratios of mixed hydrophobic/hydrophilic were also synthesized. For laboratory scale testing, the functional polymeric coatings were tested with two fouling organisms (Ulva and Navicula) with extremely different fouling mechanisms as one release well from hydrophobic surfaces and the other release well from hydrophilic surfaces. These two organisms were kept as bench mark for the release properties of our functional surfaces. The objective here is to make surfaces which show good foul release for both of these foulants. The fouling release property of a surface was found to be dependent on the surface characteristics. The proof-of-principle of these multilayer systems was demonstrated and, with understanding of the key problems involved, improved systems for fouling release that eliminated the use of toxic organocopper antifoulants were developed.
A series of potential semifluorinated SABCs based on a polystyrene-block-polybutadiene-block-polystyrene (SBS) precursor were synthesized. This reaction was then successfully scaled up using the F8H2 semifluorinated side chain to produce enough SABC for small scale surface analysis and biofouling testing. Testing and characterization was performed, with somewhat mixed results. Surface characterization suggested the presence of fluorine at the surface of multilayer coatings based on SBS-Br-F8H2, and biofouling assays showed some encouraging results—with lower settlement and growth of both barnacle larvae and Ulva spores and sporelings. Quaternized polymers used for many antibacterial applications were also used for this project for biofouling applications. Although these polymers showed very good results for protein fouling resistance they did not show good results for biofouling tests with Ulva and Navicula.
Subsequently, SABCs based on perfluorinated alkyl and polyethylene glycol (PEG) groups attached to polystyrene-block-acrylic acid (PS-b-PAA) were synthesized. These materials show either hydrophobic or hydrophilic properties. PEG, used for many biofouling related applications to proteins, was selected as the hydrophilic component.
A soft triblock based SABC was used instead of the diblock copolymer to improve coating robustness. These polymers showed good results for all standard biofouling organisms including Ulva, Navicula and barnacles. These polymers included different functional side chains such as PEG, perfluoroalkyl groups, mixtures of PEG/perfluorinated segments, Brij, Pluronic, and mixtures of hydrocarbon units with PEG attached to triblock copolymers. Many of these polymers showed excellent results against Ulva and Navicula. The mixed amphiphilic polymers were of great interest because the ratio between hydrophilic/hydrophobic properties can be easily varied by synthetic methods.
All of the materials produced have been characterized thoroughly for composition. The surface characterizations of these materials were also performed and their surface properties showed good agreement with the observed biofouling test results. All surfaces showed rearrangement in water with the hydrophilic groups migrating to the surface over time. In general, it was found that the amphiphilic and the “mixed” surfaces showed extremely good results for biofouling control (fouling release) and it appears that the desirable behaviors can be obtained.
SABCs were applied to the surface using a simple spray coating technique and can certainly be applied to large surfaces in an industrial scale. Uniform surfaces were prepared by this method. The SABCs were spray coated on a soft polystyrene-block-ethylene/butylene-block-styrene (SEBS) layer. A particular SEBS polymer was found to have the similar elastic modulus as that of polydimethyl siloxane (PDMS) and was used as the base layer. The biofouling release results showed that the softer base actually enhances the fouling release properties of the surface. All steps used in the preparation of multi layer coatings were repeated several times for reproducibility with different annealing conditions and solvents. Suitable surface preparation steps were identified to make sure that the functional groups are on the surface.
This project has identified environmentally benign, durable, and effective antifouling coatings that will reduce toxic metal pollution from DoD and commercial vessels in domestic and foreign ports without sacrificing antifouling performance. The studies performed under this project have also enhanced the fundamental understanding of fouling release behavior.