In-depth | COATINGS
Figure 5: Comparison of fluoropolymer- modified vs. hydrogel modified PDMS- based FR paints (fouling growing from the panel edges should be disregarded). The picture has been taken after 130 weeks of static immersion in the Mediterranean Sea, characterised by strong algal fouling pressure (Hellio and Yebra, 2009; Chapter 16). More examples of performance can be found in Thorlaksen et al. (2009).
Epoxy coatings are chemically curing
(i.e. reacting) systems and, therefore, shrink considerably upon reaction (Figure 7). Tis creates internal stresses which are likely to cause cohesive detachments of the topmost layer of the aged antifouling, especially if the surface preparation (i.e. washing) has not been done properly. As a solution to this problem, the patent- protected NEXUS X-Seal is formulated as a modification of Hempel’s epoxy-silicone tie-coat NEXUS by, among other changes, the addition of physically drying resins in order to maximize its flexibility (Figure 7) and compatibility with AF coats, while maintaining its strong adhesion to silicone topcoats. Figure 8 compares a full blast scenario
Figure 6: Comparison of traditional specification when converting an antifouling-coated hull to silicone (left) to the new solution using NEXUS X-Seal. Applying epoxy over old AF can cause cohesive failure of the weak topmost layer of the aged antifouling. The latter is unlikely to happen when more compatible and flexible coatings such as NEXUS X-Seal are used.
PDMS coating reinforced with hydrogel moieties overperforms the amphiphilic fluoropolymer approach.
Commercial tie-coat technologies Until very recently, the application of a FR system on a hull required as the first step the removal of previous paint coats. Tere is no need to say that the results shown in Figure 4 only have practical relevance if the Fouling Release topcoat is able to maintain its integrity and adhesion to the anticorrosive system throughout the entire dry docking interval. Hence, mastering the tie-coat technology has been a major milestone in the commercial expansion of FR systems. Table 3 summarizes the main tie-coats currently used to bridge epoxy undercoats to FR topcoats. From Table 3 it can be seen that
only one product is not based on the polydimethylsiloxane (i.e. silicone elastomer) chemistry. Compared to
22
silicone elastomer products, NEXUS’s patented epoxy-based chemistry provides a very strong chemical adhesion to the epoxy anticorrosive coats (very much like epoxy-epoxy adhesion). In order to safeguard strong adhesion to the silicone topcoat, the epoxy backbone is further modified with highly specific and reactive anchor groups.
Sealing aged antifouling coatings As an alternative to full blasting, a few marine paint suppliers currently use standard coatings from their assortment to carry out the conversion from antifouling to Fouling Release. Since standard silicone- based tie-coats do not adhere to old antifouling coats, the first step within the industry has been to seal the old antifouling by means of an epoxy coating followed by the standard tie-coat (Table 4 and Figure 6). As elaborated below, using epoxy coatings as “link-coats” poses some risks.
to a scenario in which NEXUS X-Seal is used instead. It is obvious from this figure that full blasting operations are very time consuming and constitute a large percentage of the extra time in dock compared to the sealing alternative. Compared to the standard lincoat+tiecoat conversion procedures, NEXUS X-Seal poses a lower cost, as it saves one full coat and reduces time in dock. As shown in Figure 8, using X-Seal brings the dry docking cost to the level of a conventional antifouling.
Touch up and repair of silicone topcoats One of the drawbacks of silicone-based coatings stated in Yebra et al. (2004) relates to the difficulties in making fresh “non-stick” coating coats stickto old paint systems. Blasting damaged areas results in a transition zone at the edges between the newly exposed steel and the old FR topcoat. Tis area consists of a range of different substrates, each of them exposed at varying degrees depending on the hydrojetting process. A sample of the above mentioned “transition” areas is shown in Figure 9. When conventional tie-coats are used to repair these areas, their adhesion to these old coatings exposed at the edges of spot repaired areas is clearly insufficient, which may result in detachments as shown in Figure 9 (right). NEXUS X-Tend is the only tie-coat in
the market designed specifically for touch up and repair of silicone vessels. It features excellent adhesion to the broad range of substrates observed in Figure 9. As a
The Naval Architect January 2011
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72