The Naval Architect: June 2019
Marine growth can decrease ship performance drastically, resulting in a 30-50% increase of fuel consumption. Hull fouling is also responsible for the spread of invasive species, even more so than ballast water, making anti-fouling 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 rebuild the protective boundary layer with new toxins. As the hosting paint itself dissolves in water, the surface of such ‘self-polishing copolymers’ remains fairly smooth.
After five years, the paint and its embedded biocides have typically been exhausted and the ship is recoated in drydock. Since the 2008 ban on TBT, copper compounds have become the predominant biocide. Various herbicides and fungicides are added to address plant fouling, which is not affected by copper compounds.
These additional toxins are dubbed ‘boosters’ and some (including Irgarol 1051 and Diuron) have come under scrutiny, resulting in regional legislation to curb their use. As such, copper- and biocide-based anti-fouling paints are now widely seen as a bridging technology. Leaching copper and micro-plastics (the dissolved ingredients of today’s standard SPC coatings) into oceans is not sustainable. The way forward is to phase out biocide-based paints and to adopt non-toxic alternatives.
The Teflon principle
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 faster craft the surface may be self-cleaning, however, cleaning is necessary on most 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 dockings.
‘Nano-coatings’, meanwhile, use bio-inspired microscopic surface structures (e.g. shark skin, lotus effect, etc.). Several such products are already on the market, but research continues.
New ideas in the wings
Could 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.
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.
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. But already, transducers are a very attractive complementary technology to protect recessed areas, such as cooling water pipes or sea chests. A strong point of this approach is that it offers biocide-free protection for ships even at zero speed.
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.