Volker Bertram, of Stellenbosch University’s Department of Mechanical & Mechatronic Engineering, paints a picture of the latest developments in coatings technologies.
Fouling has been a headache for shipping since ancient times. An Aramaic scroll dated from around 400 BC stated: “the arsenic and sulphur have been well mixed with Chian oil … with the mixture evenly applied to the vessels' sides so that she may speed through the blue waters freely and without impediment.” Oh well, the ancient Egyptians tried. So did Columbus and Horatio Nelson while today’s top operators continue to test the latest solutions. Each generation has come closer to the ultimate goal than the last, but none have been entirely successful or satisfied in their antifouling endeavours.
Marine growth can decrease ship performance drastically, resulting in a 30-50% increase of fuel consumption (and associated emissions) compared to a smooth hull. Hull fouling is also responsible for the spread of invasive species, even more so than ballast water. Anti-fouling, i.e. any measure to prevent or reduce fouling, is thus both an economic and ecological necessity.
Biocide paints just a bridging technology
The era of modern anti-fouling paints began in the 1940s. When in contact with seawater, such paints release biocides which form a toxic boundary layer, preventing marine growth. As the ship moves through water, the toxins are washed off and the paint must re-build the protective boundary layer with new toxins. As the hosting paint itself dissolves in water, the surface of such ‘self-polishing copolymers’ (SPCs) remains fairly smooth. After five years, the paint and its embedded biocides have typically been used up and the ship is recoated in drydock.
Since the 2008 ban on highly effective TBT biocides, copper compounds have become the predominant anti-fouling biocide. Various herbicides and fungicides are added to address plant fouling, which is not affected by copper compounds. These additional toxins are dubbed ‘boosters’. Some of the boosters (including Irgarol 1051 and Diuron) have come under scrutiny, resulting in regional bans or legislation to curb their use. As such, copper- and biocide-based anti-fouling paints are now widely seen as a bridging technology. But what could be on the other side of the bridge?
We are moving slowly yet steadily towards sustainable shipping. Leaching copper and micro-plastics (the dissolved ingredients of today’s standard SPC coatings) into the world oceans is not sustainable. The way forward is to phase out biocide-based paints and to adopt non-toxic alternatives. There is no shortage of ideas, but the road from concept to deployment is often a long one. This is especially true for antifouling, where a product’s success and effectiveness are generally measured over five years, the standard docking interval.
The Teflon principle may not stick much longer
The surface energy is a measure of how easy or difficult it is to stick to a material. Low-surface energy coatings (LSE) – foul release or silicone coatings – use the same principle as Teflon pans: making adhesion (of fouling organisms) difficult. Even if fouling is not completely prevented, such ‘non-stick’ coatings are much easier to clean, e.g. by wiping or low-pressure rinsing. On speed boats the surface may be self-cleaning, however, cleaning is necessary on most other ships, especially in niche areas such as bow thruster tunnels and sea chests.
LSE coatings, like Teflon, are mechanically sensitive and fouling starts rapidly after the coating has been scratched. Even if the coating avoids surface damage the silicone film weathers over time, rendering it less effective. While the star of classical silicone coatings seems to be waning, with some new twists the idea lives on.
Silicone coatings are super-hydrophobic (i.e. water repellent). At the other extreme, super-hydrophilic (i.e. water attractive) surfaces also impede fouling. Such ‘hydrogel’ coatings are akin to soft contact lenses. Many fouling organisms mistake the surface of a hydrogel for water; in other words, the hull surface becomes invisible for them. Combined with a mechanism to trap biocides on the hull surface, this approach can reduce biocide leaching by a factor of 10-20 over conventional anti-fouling coatings with virtually constant performance between docking intervals. Hydrogel solutions are offered by at least two major coating manufacturers.
‘Nano-coatings’ use bio-inspired microscopic surface structures (e.g. shark skin, lotus effect, etc.). Several such products are already on the market but current research is continuing to present new ideas. The Fraunhofer institute in Hamburg, for example, is developing new coatings mimicking floating ferns, which trap a fine film of air.
You have seen swimming pools cleaned by robots. You have seen lawns mowed by robots. Couldn’t robots clean ships every time they are in port too? Yes, they could. But the coating should be adapted to it and current hull cleaning robots must learn a few more tricks, most notably team work.
Biocide-based antifouling paints release toxins under shear forces. Thus, any brushing or wiping will release more toxins, leading to premature degradation of the coating. Hard coatings, on the other hand, can endure frequent cleaning (e.g. every 1-2 weeks). While the coating technology is in place, more work is needed to develop cheap, fast and widely available cleaning. And robotics is the key to this.
Over the past few years, most leading industrial nations have developed robots for underwater hull cleaning, capable of cleaning upside down under ship bottoms, handling curved surfaces at bow and stern, and recessed areas such as bilge keels. Although the technology is available, it needs to be rolled out and made widely available at competitive prices. But time is on the side of robotics, and so is industry interest.
New ideas waiting in the wings
Ultrasonic vibrations cause very high accelerations, which destroy the cell structures of algae and weed. The technology has progressed from research to industrial applications. So far, ultrasonic antifouling requires oscillators (‘transducers’) every 6-8m. For a cargo-ship, this would mean hundreds of transducers, many in areas that are difficult to access. Reliable operation in double bottoms filled with water or fuel may need more research.
But already, transducers are a very attractive complementary technology to protect recessed areas, such as cooling-water pipes or sea chests. These areas are difficult both to paint and to clean, an issue that will become an even larger headache when currently regional biofouling management regulations (e.g. for Australia or California) are applied on a wider global scale. A strong point of this approach is that it offers biocide-free protection for ships even at zero speed. Feedback from pioneering operators is mainly positive.
The “young challengers” are maturing with a growing number of in-service reference applications preparing the ground for wider acceptance. Biocide-based coating will remain king for years and maybe decades to come, but vendors and buyers are getting smarter and “performance” is being monitored by both sides.
The leading coating vendors have just begun to tap into their data treasures on how each coating performs where under which conditions, yet already we can see that this new approach is impacting the market dynamics. Performance monitoring insight allows better decisions on coatings and maintenance, ultimately fostering faster evolution to better hull management on a global scale.
Source: The Naval Architect, June 2019 (Digital Edition)