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Engineered structures deployed subsea such as offshore rigs, jetties, marine platforms and ships are under constant attack from the harsh marine environment. Protection from the influences of saltwater, biological attack, and temperature fluctuations is vital to ensure that these structures do not fail in their service life. This article specifically focuses on biological fouling of subsea structures.
Hydrophobic: Molecules or surfaces that cannot readily bond with water or other polar molecules.
Hydrophilic: Molecules or surfaces that can readily bond with water molecules through hydrogen bonding.
Amphiphilic: Molecules or surfaces that simultaneously possess both hydrophobic and hydrophilic properties.
What is Fouling?
Fouling is the accumulation of unwanted material on a solid surface to the detriment of function. Biofouling can be split into two distinct types; macrofouling, which includes large plants and animals such as barnacles, mollusks, tubeworms, and green weed, and microfouling, which is comprised of unicellular algae and bacteria also known as slime. On a ship’s hull this increases hydrodynamic drag, lowering the maneuverability of the vessel and increasing fuel consumption.
In heat exchangers, biofouling can reduce efficiency, clog systems and cause localized hot spots resulting in catastrophic failure. Biofouling also reduces the accessibility of steel and concrete surfaces for corrosion inspection. This ultimately leads to increased costs due to increased fuel, maintenance and lost time. Once biofouling has occurred, removing it can be difficult and expensive and risks damaging the protective coating beneath the fouling.
History of Anti-Fouling Coatings
Though there are a variety of methods for inhibiting organic and inorganic growth on wetted surfaces, the majority are a form of protective coating. Toxic antifoulants were historically used to controlling biofouling. Paints containing biocides such as lead, arsenic, and mercury were applied to the underwater substrate (such as a ship’s hull), which would release slowly and reduce fouling by killing the organisms. These coatings would also harm non-target aquatic life, which caused the use of them to be banned.
Self-polishing copolymer (SPC) paints were a revolutionary new technology introduced in 1974. As the name indicates, the polymer dissolves away during normal vessel operation releasing the compound tributyltin (TBT) and also making the surface smoother.
TBT is very lipid soluble, so it is easily taken up by cells where it inhibits energy transfer processes in respiration and photosynthesis. However, SPC paints were banned worldwide by the International Maritime Organisation in 2003 due to the effect on non-target organisms, resulting in severe deformities in shellfish and the accumulation of tin in ducks, seals and shellfish.
This ban has created a gap in the market for environmentally safe alternatives. Foul release coatings (FRCs) are a novel technology that function due to low surface energy that degrades an organism’s ability to generate a strong interfacial bond with the surface. They effectively make the treated surface extremely smooth, slippery and low friction.
Once the velocity of the fluid is moving beyond a critical value (typically 10-20 knots) the organisms are dislodged. FRCs are a purely physical deterrent to biofouling that provide a passive, environmentally acceptable alternative to biocide-based antifoulants.
Silicone Based Coatings
Most current commercial foul release coatings are based on poly(dimethylsiloxane) elastomers (PDMSe), a silicone-based organic polymer. These hydrophobic polymers show a low adhesion of polar molecules (including adhesive proteins) and low surface energy as desired, making it more difficult for organisms to adhere to the surface.
However, this low surface energy also means that the coating itself is difficult to bond to the substrate without a tie coat. Durability is low as the coating can come away from the surface relatively easily. The PDMSe coating only functions adequately at speeds above 10 knots, at which point the organisms become dislodged and fall off the surface. This means the coating would be unsuitable for use on the hulls of ships that spend long periods of time stationary in port.
PDMSe foul release coatings are generally very effective to control macrofouling, but less effective at preventing microfouling build up due to the purely hydrophobic nature. This is explained in the subsequent section.
Extensive research has been conducted on innovative fouling release technologies to address the problems with silicone based coatings. Fluoropolymer chemistry is the utilization of polymers containing fluorine atoms, and is one of the latest advances in foul release coating technology. It creates an amphiphilic surface; marine organisms prefer either a hydrophobic or hydrophilic surface depending on the organism, so a surface that is completely hydrophilic or hydrophobic (the silicone based coating mentioned above) will create a preferential settlement for certain fouling organisms.
Researchers at AkzoNobel have demonstrated that the amphiphilic nature of the advanced fluoropolymer coatings provided better macrofouling and microfouling inhibition than silicone-based foul release coatings.
This was established in a series laboratory and field tests. Comparing silicone-based and fluoropolymer-based coatings, slime required a water flow of > 6.7 m/s for the complete removal using the silicone-based coating, compared with 1.5 m/s for fluoropolymer-based.
Over 40 weeks of testing, the percentage of slime fouling for fluoropolymer was ~10%, whereas the silicone-based coating was ~100%. Macrofouling adhesion strength was also tested, silicone-based coating gave a barnacle adhesion strength of 62 kPa compared with 34 kPa for the fluoropolymer coating.
A study conducted by Newcastle University analyzed the percentage covering of barnacles, giving 14% for the silicone-based foul release coating and ~1.8% for the advanced fluoropolymer coating. The fluoropolymer coatings also exhibited a longer service lifetime than the silicone based coatings. The silicone coatings have a high affinity to water and a limited service life; development of fluoropolymer foul releasing coating technology has addressed this.
Fluoropolymer coatings are at the cutting edge of antifouling technology, and still being developed now. For more information on the advanced fluoropolymer coating and test results see CORROSION 2014 paper no. 4468, “Novel Slime Release Biocide Free Technology,” by L. Tyson and I. Fletcher.
Materials Performance, 2015. Advanced Fluoropolymer Fouling-Release Coating. [Online]
Available at: http://www.materialsperformance.com/articles/coating-linings/2015/08/advanced-fluoropolymer-fouling-release-coating
[Accessed 9 July 2016].
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