The market offers different technologies to overcome this type of inconvenience. The article will refer to a solution that contributes to removing embedding effectively.
by Stefan Olsen*
The fight against scale: Vessels are under constant attack from the marine environment and need to fight against the accumulation of organisms that adhere to the hull. A dirty hull affects its underwater shape, increasing drag and fuel consumption.
Historically, toxic anti-fouling agents such as lead, arsenic and mercury were used to control scale but these were banned long ago due to their adverse impact on the environment. Self-polishing systems were then introduced by many, incorporating the impregnant tributyltin (TBT), but they were also eliminated due to the deformities found in mollusks and the bio accumulation of tin detected in fish and other marine species. Since the tbt ban in 2001, attention has been focused on developing anti-fouling solutions that are more environmentally friendly.
Silicone solutions
In the 1970s a firm relationship was determined between the adhesion of marine bacteria and the surface tension of the substrate. Silicone surfaces were shown to exhibit superior properties against fouling due to their high hydrophobicity (their propensity to repel water molecules). Silicones are also inherently flexible and the combination of low surface tension and flexibility was found to minimize the adhesion of marine bacteria. This technology allowed the first non-stick coatings to be patented, even if they were not fully effective.
In the late 2000s, Hempel launched its Hempasil X3 product which focused on technology to encourage the formation of an extremely hydrophilic hydrogel on the outermost surface of the coating. Although seemingly contradictory, the company understood that hydrophilic surfaces (those that attract water molecules) were more effective in preventing scale than hydrophobic surfaces, so this new technology provided state-of-the-art anti-fouling properties.
Hydrogel layers
Hydrophilic technology is effective because the hydrated layer of hydrogel polymers traps water in a gradient of liquid water to a substance more similar to that of a gel. This gradient presents a surface for bio-fouling organisms that is unlike any other that has been found. They do not recognize it as a suitable surface on which to fix and if they try, they cannot displace the water molecules attached to the hydrogel layer with their adhesion. And if they are successful in applying what binds them, the silicone-based matrix under the hydrogel layer offers very low surface energy to anchor adhesion. Together, these technology axes combine to offer a highly effective scale defense solution.
Theoretically, improving the efficacy of this technology could be achieved with the introduction of a biocide but, until recently, that had not been possible. If large quantities of biocides are introduced, the surface of the coating loses its softness and the silicone layer loses its low surface energy properties. Additionally, the release of the biocide from the coating may not be controlled and would reach a peak when the coating was first applied and then disappeared as it aged. Because of this, it was quickly demonstrated that the addition of biocides to a commercial scale detachment product did not improve its performance when compared to a biocide-free non-stick coating.
Changing the game
The game-changer came at the beginning of this decade with the introduction of Actiguard technology, which enables the effective use of biocides within a silicone-based coating.
This technology works by forming an activated hydrogel with biocide on the surface of the active defense coating against scale. When the biocide diffuses out of the film, it becomes trapped in the hydrogel layer which increases its surface concentration and prolongs the time the biocide is retained on the surface of the coating. This means that less biocide is needed and that its effectiveness in preventing the settlement of bio-fouling organisms is greatly improved.
Additionally, it also means that the biocide concentration is maintained at a level where the silicone coating retains all its anti-fouling and smooth surface properties.
Effective use of biocidal products
Actiguard only requires a very limited amount of biocide to work effectively and therefore the biocide content of the latest Actiguard scale defense system (Hempaguard) is about 5% of that value in a comparable conventional specification. Similarly, the release of biocide is much more controlled than other coating systems since the mechanism responsible for controlling the rate of release is the diffusion of the biocide through the coating matrix.
This is completely different from traditional coatings where the biocide is released on the pigment front during the self-polishing process. Actiguard is even more effective when compared to the release of biocides from non-polishing matrices. In this case, the biocidal release rate of that technology is higher and more stable, which ensures a more efficient use of the biocide while maintaining, at the same time, a helmet free of scale for longer.
Changes in temperature
In general, the speed of biocide release will vary with temperature, and vessels protected with Actiguard will get a release of more biocide as the water temperature rises. However, since scaling activity tends to increase with water temperature, an increase in biocide release is favorable. This means that the technology in question is using biocide effectively. Conversely, at lower temperatures when organisms are less active, the release of biocide from Actiguard will be correspondingly lower. No excess biocide shall be released unnecessarily.
Browsing speed
Another advantage not shown in other anti-fouling solutions is that this technology retains its stable release of biocides regardless of the speed at which the vessel moves. Extensive tests show that for speeds above 0.2 knots, the biocide continues to be released at the same rate. Even when the boat is static and where the tidal current is running at more than 0.2 knots, it is still effective. The release of 85% is still achieved at 0.1 knots. This is particularly important for vessels that generally operate at slower speeds or for those whose operating patterns require long periods of inactivity. Traditional anti-fouling agents will release more biocide when the vessel sails at higher speeds, but at these high speeds there is much less fouling activity. Therefore, from an environmental perspective, it is much kinder to the environment, as it will continue to release a stable and low amount of biocides, even at high browsing speeds.
These attributes give Hempaguard much more flexibility than other solutions. While it is necessary to specify the speed and type of operations, as well as the type of water when selecting a traditional anti-fouling coating, with Hempaguard this is not necessary, as it retains its effectiveness at all speeds and at any temperature. This means that the vessel can change its route pattern without affecting the life of its coating. Extensive testing has shown that Hempel's technology remains effective when a vessel is idle for up to 120 days and, more importantly, the coating achieves an average fuel saving of 6% when compared to a conventional anti-fouling product.
The fight against fouling has come a long way since the days of copper coating and heavy metal products. Remaining at the forefront of research and development is vital for all coating suppliers if they want to continue to develop increasingly effective, flexible and reliable solutions for their customers.
* Stefan Olsen – Manager Fouling Release Systems, R&D, Hempel A/S.
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