This is an attempt to explain buzzy like words self-polishing, depletion coatings and all those things that might sound strange, but actually this is an effort to explain and give an insight and translate intangible things to techniques to select your hull coating technology.
Actually, the performance of the vessel is analogous to:
- Its shape;
- Its surface characteristics;
- The MicroRoughness;
- The MacroRoughness;
Apart from the shape of the hull all the other parameters can be saved during the operation of the vessel, so this is what we have to focus on. Just keep in mind that a change in roughness of about 10 to 20 microns during the operation of the vessel might increase by 1% the total hull resistance.
The hull is a metal surface, so it has two major threats:
- The physical corrosion
- Biological Fouling
Especially when it comes to antifouling it takes only one minute by the immersion of a metal surface inside the sea, for the first organic molecules to attach to the surface and initiate the process of the fouling. By 24 hours we have bacteria, and by three weeks we have the hard fouling, or barnacles.
The fouling is a huge problem. It has an immense economical effect on the operation of the vessel. If there was no antifouling system, the 150 kilos of sea grove would attach to the surface of the hull. The loss of speed for moderate fouling can go up to 15 or 18%, and a medium containership can save up to $1,000,000 per year just from protecting its hull. In essence it’s ‘Paint now or Pay later’. So, the existence of antifouling system is essential for the good operation of the vessel.
The Essential Check List
It’s not only about economical things, it’s also about emissions, and also about invasive species; a biological problem that can be transferred all over the world. So when a director is about to select an antifouling system, this the major checklist points to address:
- Idle Promise;
- Sailing Area;
Fouling & Sea Patterns
Fouling patterns are the high risk areas of fouling all over the world, which are almost identical with the same patterns of the current vessels. The sea has a rather stable salinity, ranging from 3.4 to 3.5%, up to a depth of 4,000m. There are some areas of extreme or low salinity, maybe the ice areas. The temperature presents large variations on latitude, from -2 in ice areas to even +30 tropical areas and the diurnal variations do not exceed 0,4. These are physical constants and data all over the world.
The Paint Essential
A paint is a mix of four main components:
- Functional Additives;
- Pigments & Fillers;
One component that it is very important is the solid content of the paint. A solid content of the paint is everything that will stay on the surface after you have evaporation or turing of the paint. The solid content is important because if all coatings are measured in terms of dry film thickness, the more solid content is in the paint, the thicker the coating will end.
All antifouling, especially the self-polishing antifouling system contain biocides. After the ban of tin compounds in nearly 2000, the major biocide component of antifouling paint is Copper, and actually it’s Copper 1. Copper has broad range, low cost, but it has to be overloaded inside the paint, in order to give good results. Actually, there are some anti-bacteria or antifouling biocidal components and there is a new trend of very high specific that have been developed especially from pharmaceutical companies, in order to provide antifouling properties in long tidal conditions.
- A good antifouling paint has more than 30 to 35% of Copper inside. So for every 100 grams of the paint, 30 or 35 grams are Copper.
- New antifouling biocides are performing very well, especially in idle conditions and whenever you meet a component called Copper Pyrithione, it helps Copper perform better.
Antifouling coatings lose some of their thickness during the voyage of a vessel, and in this way they lose their thickness and release the biocides which are dissolved in a hardly- soluble oily matrix (rosin) and blended in insoluble polymer coating.
There are 3 different mechanisms: In the Control Equation, you have a rosin inside, a pipe oil for drowsing that gets slightly released to the environment in order to release the biocides. Some more modern technologies include the Silyl Acrylate and the Copper or Zink Acrylate. These are the backbone, the binder that holds all the components together. The property of hydrolyzing, meaning that they break down, they become soluble to water, so even the binder gets dissolved in the water and in this way the antifouling coating, loses some of its thickness.
The major difference between the Silyl Acrylate and the Copper or Zink Acrylate, is based on some silicon molecules that hey break down. They present a low erosion rate, but they perform poorly especially against the diatoms. So many Silyl Acrylate coatings carry slime along the pockets of the vessel.
From the other hand, Copper or Zink Acrylate they are a bit better, but the problem is that they need salt, in order to get dissolved. So they do not perform very well especially next to fresh water areas.
Another technology that developed in the last decade is the fouling release coating. Those are coating that are very slippery and they do not allow a strong adhesion of the fouling to the surface. Fouling release coating will never become antifouling coatings, but due to the friction and the movement of the vessel, due to the fact that the adhesion is poor, then the fouling goes away. There are some certain surface properties that can help you fine tuning this fouling mechanism. They are more resistant to cleaning, but they are very hard to apply and to remove.
The game changer in antifouling coating will be in the years, the roughness. The roughness is not only because it will reduce the friction coefficient and reduce the fuel consumption, is because roughness will present better antifouling performance. The two bit pattern in roughness coating means that the water can become stagnant in in centered areas and these areas will present fouling nuclei, the fouling will start from there. So the important thing is to drop below 40 or 50 microns in initial roughness, in order to have a better antifouling performance.
Closing some tips in order to select a better antifouling system:
Binder or Biocide?
- Binder technology is completely different than biocide activity.
- A high tech binder system would perform poor with low Copper 1 loads.
- Use paint schemes that combine different binder technologies.
- Low thickness (DFT) reduces cost and useful lifetime expectancy.
- Minimum DFT for 60M performance is 250-300μm.
- Extended lifetime performance promise with low thickness will result to inferior AF performance.
- “Old fashioned” rosin based AFs perform better in long idle periods and demanding sailing patterns.
- Avoid Zinc/Copper Acrylates next to freshwater environments.
- Underwater cleaning before Dry-Dockings saves huge on blasting!
- Hull performance monitoring systems do pay off.
The views expressed in this article are solely those of the author and do not necessarily represent those of SAFETY4SEA and are for information sharing and discussion purposes only.
Above text is an edited article of Ioannis Arabatzis' presentation during the 2018 GREEN4SEA Conference
View Ioannis Arabatzis’s presentation on Hull Coating Technologies during the last GREEN4SEA Conference herebelow
Dr. Arabatzis has dedicated his professional track inventing clever materials that solve every day problems. After winning the 1st prize in the European Business Plan Competition held in Stockholm in 2005, he is the founder, managing director and legal representative of NanoPhos SA, a company that develops nanotechnology driven formulations for satisfying the needs of the marine industry. Dr. Arabatzis has organized the R&D department of NanoPhos SA, as the primary think/experimentation tank for New Product Development. Apart from his research experience, he is interested in quality and process management, green legislation issues (especially in the field of nanotechnology), economic impact of nanotechnology and entrepreneurship in nanotechnology. As a chemist, Dr. Arabatzis is interested in developing functional coatings for the marine industry, solving old problems with new ideas. Coatings that respond to external stimuli, self-cleaning/healing technologies, antifouling strategies and friction reducing innovations. He is inventor in 3 patents, author of 1 book and co-author of more than 25 scientific articles, with more than 1000 cited references.