In-depth | COATINGS Smooth operator
In the second and final part of our fouling control paper Hempel A/S experts Diego Meseguer Yebra and Pere Català examine the use of non-toxic, fouling release (FR) technologies.
A
nyone who has ever fried an egg can testify to the usefulness of non-stick technology, but in the
marine world anti-fouling coatings can save more than just breakfast. Cost savings on coatings that maintain a smooth hull surface can see significant savings on both fuel and emissions.
Non-stick, technology During the transition from tin-based antifouling products, it was difficult to foresee the commercial success that silicone-based, biocide-free topcoats have experienced in the past few years. In reviews such as that by Yebra et al. (2004), the fouling release technology was just described as a “promising” environmentally friendly option:
• High solid content (i.e. reduced solvent emissions)
• One topcoat required compared to two - three coats for biocide-based
• Completely free of organic biocides and heavy metals (except for trace amounts
anti-fouling coatings. Tis translates into time-savings in the yards lower solvent emissions and reduced consumption of synthetic materials (lower carbon footprint)
• Low hydrodynamic friction with seawater, resulting in reduced emissions
• No film degradation and no significant leaching of any ingredient into the water
of CO2 matter
column (Finnie and Williams, 2010). In biocide-based paints, the entire formulation is either dissolved or eroded
1 Assuming sixty million litres per year of antifouling paint.
Only topcoats have been taken into consideration. Te average dry film thickness of the AF paints is estimated to be 300μm (e.g. two layers of 150μm) compared to 150μm for the FR topcoats.
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of catalyst). Hence safer for marine life and reducing toxic waste at dock
, SOx, NOx and particulate
Figure 1: Simulated accumulated fuel savings for a Fouling Release coating vs. a 100% clean self-polishing copolymer paint as a function of progressive fouling settlement on the FR coating (10% self-smoothening is assumed on the SPC: Weinell et al., 2003). Simulations run for a 7000TEU container vessel burning 170tonnes of fuel/day and with an activity of 80% (US$470/tonnes of 380-cst bunker fuel).
by seawater (i.e. released into the water column).
Quantifying the above facts, a
hypothetical complete conversion to FR coatings would save the environment more than 20million litres of solvent, 70million tonnes of Cu2
of organic biocides annually worldwide1
O, and six million tonnes .
In spite of this compelling profile, a few inherent drawbacks related to the silicone
chemistry were believed to jeopardize its commercial success. However, two main factors are responsible for the new reality of this technology. On the one hand, the leap in performance that these technologies have experienced in the past years (see the section below) allowing them to expand to most of the trading fleet while reducing their risk profile and cost. On the other hand, overwhelming scientific evidence showing that freshly
The Naval Architect January 2011 Source Weinell et al. (2003) Candries et al., (2003) Schultz (2004)
Candries and Atlar (2005)
Westergaard (2008) sCF% 6.1% 3.5; 3.0–4.0% 5.3% Remarks
Rotary study. Topcoat on smooth PVC
Rotary study. Full system on smooth PVC
Full system on 304SS. No sandpaper strip
Topcoat on smooth steel. Turbulent boundary layer measurements
1.4%
Towing test. Full system on smooth Al/smooth undercoats
5.0%
Towing test. Full system on Rz50
467µm panels Table 1.
Representative differences in friction coefficient ( CF) when
Δ
comparing clean Fouling Release coatings to self- polishing type ones. Non-fouled silicone topcoats are reported to consistently decrease the drag resistance of a hull compared to eroding-type paints.
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