In-depth | COATINGS
Figure 3: Performance of a hydrogel- modified PDMS topcoat against slime, large size hard fouling and algae growing on inert areas after 10 months sailing in waters with a high fouling pressure (left). Hydrogel Silicone performance against a self-polishing AF on the hull of a bulk carrier sailing around Europe (right).
Figure 4: Results obtained from fast screening assays performed on early Hydrogel Silicone commercial coating compared to the standards. a) Settlement of C. lytica. b) Settlement of N. incerta. c) 2 week reattached barnacle adhesion test with B .amphitrite (measured/broken). The two first reference systems correspond to a commercial pure PDMS-based technology and a commercial PDMS + fluoropolymer technology respectively.
Back at the turn of the 21st century, very
• Te important R&D investment made by marine paint manufacturers in designing
few vessels had been applied with silicone- based FR paints. Tere were three main hurdles at that time for this technology:
• Few customers were ready to pay the higher cost of the new silicone paints
their tin-free antifouling assortment to replace TBT-SPCs
• Concerns with adhesion of silicone topcoats to anticorrosive systems (see
including costs of full blasting
Commercial tie coat technologies below). Tese topcoats, on the other hand, had
issues of staying fouling free so they were only specified for fast moving vessels (e.g. above 15knots or even more) which sailed most of the time (e.g. above 75% activity). While these stringent requirements limited the settlement of macro-foulers such as algae and barnacles, they were not enough to prevent the colonisation of the paint surface by diatomaceous slime or its release during sailing. As an improvement over hydrophobic
PDMS based coatings, the fluoropolymer- modified silicone topcoat Intersleek 900 was the first of a new range of Fouling Release coatings incorporating advanced
20
surface functionalities and allowing its use on vessels down to 10knots. As an extension to Finnie and Williams (2010), Table 2 reviews the most recent commercial products. As mentioned above, Intersleek 900 was
the first commercial product moving away from 100% hydrophobic Fouling Release formulations. Quoting Finnie and Williams (2010), fluoropolymer-modified silicone paints can be described as “amphiphilic network coatings which are designed to provide compositional and topographical heterogeneity with the aim to reduce adhesive interaction with complex marine adhesives”. Since it has been widely shown that different organisms prefer different surface energies these coatings alternate between both hydrophilic and low-surface energy hydrophobic domains, aimed at offering a broad spectrum of fouling protection. Te “Hydrogel Silicone” approach used
by Hempel rather focuses on protection against diatom and algal spore fouling settlement, the main fouling types affecting “standard” PDMS-based coatings (Hellio and Yebra, 2009; Chapter 26). It is widely acknowledged that hydrophobic domains favour the adhesion of this type of fouling (Finlay et al., 2002), while the glue of
diatoms and algal spores cannot displace the water entrapped in the hydrogel surface, hence preventing their settlement (Ekblad, 2010; Rosenhahn et al; 2010). According to Rosenhahn et al. (2010), “the hydration water on OEG (i.e. hydrogel chemistries related to those found in Hempasil X3) is similarly stable as the co-existence of water and ice at lower temperatures”. Just like previous generations of FR paints
(Hellio and Yebra, 2009; Chapter 26), the adhesion of barnacles to hydrogel-modified PDMS surfaces is also extremely low. Te efficiency of hydrogel-based FR coatings has been demonstrated in laboratory high throughput screening techniques (Hellio and Yebra, 2009; Chapter 15). Te results are illustrated in Figure 4. A consistently lower settlement of marine foulers (both soſt and hard fouling) on the hydrogel- based coating compared to the references is observed. These references consisted of
two commercial FR coatings (a “pure”-hydrophobic PDMS coating and a fluoropolymer-modified topcoat), one commercial silicone elastomer and one polyurethane coating (negative reference). Tese results correlate very well to the field performance of this coating technology (Figure 5) and shows that hydrophobic
The Naval Architect January 2011
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