It is accepted wisdom that for hull coatings to contribute to fuel saving then they must be effective at preventing biofouling either by way of biocides or foul release and the surface should be as smooth as possible to reduce friction. Since the banning of TBT most R&D has been directed at ensuring the antifouling performance is somewhere approaching that of TBT.
Smoothness is acknowledged as an important aspect, but it is not completely in the hands of the coating manufacturer as surface preparation, application technique and ambient conditions all affect the final result. More to the point, smoothness will change over time due to contact damage with quays, chains and floating objects along with the lower quality of running repairs to the coating. To be fair, the antifouling action of some coatings will also deteriorate and it is hard to know which factor has the greatest effect on performance.
There are ways of measuring smoothness – or to be more precise roughness – such as using a hull roughness analyser and taking readings from various parts of the vessel. A very good condition would have variations in surface level of around 120 microns but a reasonably well maintained ship about one third into its expected lifespan that might increase to 400 microns.
While smoothness at the micron scale is a good thing, the latest scientific research at the nanoscale suggests that roughness can be the key to improvements in ship performance. In January this year, it was reported that research project led by Professor Chiara Neto at the University of Sydney Nano Institute has developed nanowrinkled coatings with antifouling properties that do not have toxic components. Details of the project were included in a paper published in December last year by the American Chemical Society.
Carnivorous plants and sharks share secrets
The inspiration for the coating structure described as nano wrinkles was a carnivorous pitcher plant which catches a layer of water on the tiny structures around the rim of its opening creating a slippery layer that causes insects to slide into the plant’s pitcher. The material that the team developed is composed of Teflon on a Polyshrink substrate, a silicone oil is used as the lubricant.
In tests in Sydney harbour, the infused surfaces displayed stability in seawater and inhibited growth of Pseudoalteromonas spp. bacteria up to 99%. Over the seven weeks of testing the results showed that silicone oil infusion inhibited the attachment of algae, but the algal attachment increased as the silicone oil was slowly depleted over time. As well as having potential for anti-fouling, the infused wrinkled surfaces have high transparency and are mouldable, making them suited to protect the windows of underwater sensors and cameras.
More inspiration from nature and in particular sharks have been behind two EU-funded antifouling projects one of which has completed and one still ongoing. In the Seafront project which involved coatings manufacturer AkzoNobel along with several other companies and academic institutions, one aim was the biomimicry of the riblets on a shark's skin which are known to reduce hydrodynamic drag by around 5-6%. The project successfully demonstrated a riblet coating system with integrated Intersleek fouling control.
Although there is as yet no commercial development, AkzoNobel and another of the project partners, Italy-based Bio-on have agreed to continue their relationship. The aim of the collaboration is to continue to investigate the use of Bio-on’s biodegradable and bio-based polymers as components in fouling control coatings.
The second project is named eSHaRk and involves coatings maker PPG along with several other organisations including shipbuilder Meyer Werft and the Hamburg Ship Model Basin HSVA. The intention is to develop a self-adhesive, fouling release film to be used on commercial ocean-going vessels. It too uses an existing antifouling product, PPG’s SIGMAGLIDE.
While there are obstacles to overcome in developing a film that will remain adhered to the vessel and in training contractors to apply it, the fact that it would be manufactured under controlled conditions would suggest that some of the application faults of conventional coatings could be avoided.
Barnacles given the boot
One product which mimics nature and which has already notched up commercial success is I-Tech’s Selektope which is now an active ingredient in products from at least two of the major coatings manufacturers. Selektope is an organic, non-metal compound that inhibits barnacle settlement on ships’ hulls by stimulating the barnacle larvae’s swimming behavior with reversible effect.
It is characterised by high efficacy at extremely low concentrations in a marine coating (0.1% w/w), ultra-low leaching, and flexibility to boost copper-based paint formulations or replace copper completely. A secondary benefit of the compound is that it remains effective even if the ship is idle for long periods. It has been approved for use by the EU, Japan, South Korea and Chinese authorities.
Initial references were provided by Chugoku Marine Paints which in 2016 incorporated it into SEAFLO NEO CF PREMIUM and SEAFLO NEO-S PREMIUM. Last summer, the Japanese coatings maker launched a new product SEAGRANPRIX 880HS+.
In October Hempel launched two new antifouling coatings; Globic 9500M and Globic 9500S. The company claimed significant fuel savings for the self-polishing coatings the first of which is designed to protect against slime as well as soft and hard fouling in all conditions. Globic 9500S (S for static) is designed to protect against hard-fouling even during extended outfitting periods and includes Selektope as a biocide. Other claimed benefits are a high mechanical strength to avoid cracking and peeling.