Marine Paints - The Particular Case of Antifouling Paints

19
Progress in Organic Coatings 59 (2007) 2–20 Review Marine paints: The particular case of antifouling paints Elisabete Almeida a,, Teresa C. Diamantino a , Orlando de Sousa b a LTR/DMTP/INETI, Estrada do Pa¸ co do Lumiar, 1639-038 Lisboa, Portugal b HEMPEL (Portugal) Lda, Vale de Cantadores, 2951-501 Palmela, Portugal Received 6 July 2006; accepted 3 January 2007 Abstract The authors present a general overview of marine paints, paying particular attention to the case of antifouling paints. After locating these paints in the anticorrosive protection systems used on the underwater parts of ships and/or other moving structures, a summary is made of the main types of antifouling products used through history up to the present time. This is complemented by a systematic assessment of the main types of living organisms that fix themselves to the underwater parts of ships. Consideration is also briefly made of the main basic mechanisms by which the different types of antifouling paints work. Finally a number of current research lines on antifouling technologies are mentioned. © 2007 Elsevier B.V. All rights reserved. Keywords: Ship protection; Underwater ship areas; Marine paints; Antifouling paints; Performance mechanisms; Fouling Contents 1. Introduction ............................................................................................................... 2 2. Short review of marine paints ............................................................................................... 3 3. Protection of underwater ship parts .......................................................................................... 4 3.1. Interaction between ship hull and sea water ............................................................................ 4 3.2. Marine organisms ................................................................................................... 5 3.3. Antifouling technologies ............................................................................................. 7 3.3.1. First technologies used prior to mid 19th century ............................................................... 7 3.3.2. First antifouling paints used on steel hulls prior to 1960 ......................................................... 8 3.3.3. Main types of products used on steel hulls in the second half of the 20th century ................................... 8 3.3.4. Alternatives to painting ..................................................................................... 11 4. More environment- and man-friendly antifouling paints ...................................................................... 12 4.1. Tin-free controlled depletion paints (CDPs) ........................................................................... 12 4.2. Tin-free biocide-containing self-polishing paints (TF-SPCs) ............................................................ 14 4.3. Hybrid paints ...................................................................................................... 15 4.4. Biocide-free paints ................................................................................................. 15 4.5. Some recent or current developments ................................................................................. 16 5. Conclusions ............................................................................................................. 17 Acknowledgements ....................................................................................................... 18 References .............................................................................................................. 18 Corresponding author. E-mail addresses: [email protected] (E. Almeida), [email protected] (T.C. Diamantino), [email protected] (O. de Sousa). 1. Introduction It is fairly common for the area of anticorrosive protection painting (APP) to be underestimated by specialists in other areas of science, such as electrochemistry or biology. Painting, ances- trally known for its empiricism, brings a frown to the face of 0300-9440/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2007.01.017

Transcript of Marine Paints - The Particular Case of Antifouling Paints

Progress in Organic Coatings 59 (2007) 220

Review

Marine paints: The particular case of antifouling paintsElisabete Almeida a, , Teresa C. Diamantino a , Orlando de Sousa ba b

LTR/DMTP/INETI, Estrada do Pa o do Lumiar, 1639-038 Lisboa, Portugal c HEMPEL (Portugal) Lda, Vale de Cantadores, 2951-501 Palmela, Portugal Received 6 July 2006; accepted 3 January 2007

Abstract The authors present a general overview of marine paints, paying particular attention to the case of antifouling paints. After locating these paints in the anticorrosive protection systems used on the underwater parts of ships and/or other moving structures, a summary is made of the main types of antifouling products used through history up to the present time. This is complemented by a systematic assessment of the main types of living organisms that x themselves to the underwater parts of ships. Consideration is also briey made of the main basic mechanisms by which the different types of antifouling paints work. Finally a number of current research lines on antifouling technologies are mentioned. 2007 Elsevier B.V. All rights reserved.Keywords: Ship protection; Underwater ship areas; Marine paints; Antifouling paints; Performance mechanisms; Fouling

Contents1. 2. 3. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Short review of marine paints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Protection of underwater ship parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Interaction between ship hull and sea water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Marine organisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Antifouling technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1. First technologies used prior to mid 19th century . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2. First antifouling paints used on steel hulls prior to 1960 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3. Main types of products used on steel hulls in the second half of the 20th century . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.4. Alternatives to painting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . More environment- and man-friendly antifouling paints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Tin-free controlled depletion paints (CDPs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Tin-free biocide-containing self-polishing paints (TF-SPCs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Hybrid paints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Biocide-free paints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Some recent or current developments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 3 4 4 5 7 7 8 8 11 12 12 14 15 15 16 17 18 18

4.

5.

1. IntroductionCorresponding author. E-mail addresses: [email protected] (E. Almeida), [email protected] (T.C. Diamantino), [email protected] (O. de Sousa). 0300-9440/$ see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2007.01.017

It is fairly common for the area of anticorrosive protection painting (APP) to be underestimated by specialists in other areas of science, such as electrochemistry or biology. Painting, ancestrally known for its empiricism, brings a frown to the face of

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many specialists, who have nevertheless not had the opportunity to discover the complexity and the interdisciplinarity of APP in recent decades. An excellent example of this reality can be seen in the case of marine paints [15], as will be shown below. A structure like a large ship in itself offers a wide range of exposure conditions, giving rise to a complexity and diversity of anticorrosive protection situations that need to be resolved. These include areas such as the hull, deck, superstructures, fuel tanks, ballast tanks, and others. In turn the hull presents different parts with highly specic operating conditions, going from the ship bottom (permanently immersed in sea water) to the boottop area (subject to alternating immersion conditions), splash area (above the water line with the ship fully loaded), and the top sides (which are practically always emersed and exposed to the atmosphere). In view of the variety of the problems it poses, the underwater part of the ship is the most interesting area for the APP specialist. Besides the need to assure efcient anticorrosive protection painting that is compatible with cathodic protection, it is also necessary to keep the surface as smooth as possible in order to minimise drag resistance when the ship is in movement and thus reduce fuel consumption [6,7], which means the need to prevent the attachment of a wide variety of marine organisms, both plants (ora) and animals (fauna). Furthermore, in recent decades the antifouling paints applied on ship hulls must not only prevent the fouling of underwater areas but must do this in compliance with emerging regulations and legislation [810], and furthermore are required not to release biocides into the sea water. Thus, a ght against time is on (demanded by the International Maritime Organization, IMO) [10], in an ongoing search for practically miraculous antifouling products. Following a short review of marine paints in general, the present work makes a systematic analysis of the specic problem of interaction between ship hulls and sea water and the antifouling technologies used on ship hulls from historic times to the present day. Reference is also made to other alternatives to painting that have emerged in recent decades, and an indication is given of some promising ways that are currently under devel-

opment, and which may in the near future lead to antifouling paints that can be used after the complete banning by the IMO [10] of existing efcient antifouling paints that are still in use. The latter, which are the fruit of in-depth research carried out over many years, can now be practically tailor-made [1113] in accordance with the particular characteristics of each type of ship. 2. Short review of marine paints Contrary to the past, in recent decades the anticorrosive protection of steel ships starts in the shipyard, which is now normally equipped with automatic blasting and painting plants for both steel plates [14] and steel proles. Here, the different steel surfaces are blasted and painted with shopprimer [15], in accordance with the ships construction schedule. Following a suitable drying time, the prepainted plates and proles pass on to the plateshop where, depending on the ships design, they are subjected to various operations such as marking, forming, cutting and welding. After this, they are mounted, rstly in unitary blocks or pre-blocks, and later in large blocks or superblocks, which are then transported to the site where the ship is under construction (Fig. 1). Finally, appropriate surface treatments and painting procedures are applied to each of the ships specic exposure/operating areas [1]. Nowadays these blocks can be treated and tted out in blasting/painting cabins, after which they are transported to the construction site. In recent decades the paint systems used in shipbuilding have undergone enormous development in correspondence with emerging regulations and legislation, especially those related with protection of the environment and of human health. These developments have concentrated above all on reducing volatile organic compound (VOC) contents and eliminating toxic and carcinogenic components from traditional paint products. Table 1 lists several examples of typical paint systems currently used in different critical areas in ship painting. This table does not include the painting of underwater or boottop areas, which constitutes the main objective of this work and as such will be dealt with below in greater depth.

Fig. 1. Views of several prepainted blocks: (a) unitary; (b) superblock; (c) mounting of blocks in shipyard [14].

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Table 1 Examples of typical paint systems currently used in different critical areas of ship painting [1,16] Critical areas in ship painting Sides and superstructures Main requirements Appearance, anticorrosive protection, washability, UV resistance Typical paint systems Pure or modied epoxy primer/epoxy topcoat Aliphatic polyurethane Aliphatic polyurethane/acrylic Polysiloxane/epoxy hybrid (anti-rust stain topcoat may be applied) Zinc silicate followed by two-pack epoxy/polyurethane system Trowel- or roller-applied elastomer (13 mm) over high build primer Topcoat should include aggregates, e.g. aluminium oxide, silica or others to provide non-slip properties

Decks

Appearance, anticorrosive protection, washability, UV resistance, non-slip

Tanks Ballast

Resistance to alternating contact with sea water and with transported products

Modied epoxy Aluminium-pigmented pure epoxy Solvent-free epoxy Special waterborne asphaltic emulsion Cement-reinforced acrylic High-solid or solvent-free polyamine-cured epoxy High-solid polyamine epoxy Epoxy-cyclosilicone

Cargo Crude or petroleum Oils or hydrocarbons Very aggressive products

Resistance to contact with specic cargo types

3. Protection of underwater ship parts It is mandatory that protective painting systems for underwater ship parts include an anticorrosive primer and an antifouling topcoat. Sometimes a suitable tie coat is applied between the primer and the antifouling paint, especially when the anticorrosive primer contains components that may negatively affect the adhesion of the antifouling paint; e.g. in the case of primers containing coal tar, which tend to exude towards the antifouling, affecting its efciency, or in the case of short recoating intervals with products of poor compatibility with shipyard working procedures. Modern paint systems typically include a two-pack epoxy primer. Polyurethanes and coal-tar epoxy have been banned for environmental reasons. Meanwhile, in view of the fact that bre glass improves mechanical strength and water vapour impermeability, there has been a signicant increase in the use of this material as reinforcement for the aforementioned primers, especially in the boottop area of ships, which is subject to considerable mechanical forces and to periods of exposure to the atmosphere. For economic reasons, underwater ship parts must be kept relatively smooth in order to reduce drag resistance and thus minimise fuel consumption [6,7]. Accordingly, ships have long been subjected to a nal antifouling treatment. Besides assuring the necessary protection and mechanical strength, this treatment must prevent or drastically reduce the fouling of the hull, which occurs above all in port or when sailing at low speed. 3.1. Interaction between ship hull and sea water As happens to the great majority of solid surfaces immersed in sea water, after a relatively short immersion time ship hulls

become fouled with numerous marine organisms (of which there are more than 4000 species [17,18]) if nothing is done to prevent this [1921]. The degree of fouling depends not only on how long the ship remains in port or its cruising speed at sea, but also very specially on the nature of the water in the different regions of the world. For instance, it was observed in 1963 that Curacao was not a good place for studying antifouling paint performance, since the phosphate level at the ocean surface was always low, which, associated with the absence of chlorophyll, was not propitious for the xing of algae or animals such as barnacles, tunicates, molluscs or bryozoans [22]. The inuence of various sea water characteristics such as salinity, temperature, pH, dissolved salts and oxygen concentration on the fouling of immersed solid surfaces is also well known [23]. An interesting study by Pickard and Emery shows how latitude, which inuences the average temperature, salinity and density of ocean water, can affect the degree to which marine organisms become xed to immersed surfaces [2430]. This explains the diversity of fouling in different areas of the globe and why equatorial and tropical regions are richer in certain species than other temperate or cold areas. However, some species are adapting to great uctuations in environmental conditions, and are now found in numerous parts of the planet [31]. Fig. 2 summarises a series of interactions between ship hulls and sea water. In any case, it is being seen that the fouling of immersed surfaces takes place in various stages, as illustrated in Table 2. In the rst stage (rst minutes of immersion), organic molecules of proteins, polysaccharides, glycoproteins and others become physically adhered to the surface, conditioning it for a second stage, corresponding approximately to the rst 24 h of immersion, in which bacteria and unicellular algae are adsorbed. Subsequently the existence of the microbial lm that has formed

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Fig. 2. Fate of active ingredients of antifouling paints in sea water (courtesy of Hempel Portugal).

provides sufcient food to allow the xing, in a third stage, of spores of microalgae that will constitute a biolm which, in turn, will allow the increased capture of more particles and organisms, such as larvae of marine macroorganisms (fourth stage), in 2 or 3 weeks of immersion. The conditions are then set for the xing and growth of either macroalgae or marine invertebrates [1831]. 3.2. Marine organisms As has already been noted, there are a great number of marine organisms. Due to their importance in various domains of science, these organisms have been subjected to specic research. Attention is drawn to a study performed in the 1960s by the Organisation for Economic Cooperation and Development

(OECD) in many countries, including Portugal, which gave rise to the publication of a catalogue of several volumes [3235] and, among others, a French standard that systematises the practical identication of the main marine macroorganisms that x themselves to ship hulls [36]. Table 3 systematically presents certain characteristics of the main species of macroorganisms. Each of the groups and subgroups, which possess differentiated characteristics, is more or less prevented from becoming xed depending on the toxicants that have been included in the different antifouling products used over the years, as will be mentioned below. It is however known that many of these organisms do not become xed to ships that travel at speeds of above 45 knots, for which reason it is considerably easier to protect the hulls of high speed ships and those that do not remain for long in port, than the hulls of other ships.

Table 2 Stages of attachment of marine organisms on surfaces immersed in sea water Processes involved Stage 1 Essentially physical forces, such as electrostatic interactions, Brownian movement and Van der Walls forces Stage 2 Reversible adsorption of mentioned species, especially by physical forces, and their subsequent adhesion interacting together with protozoans and rotifers Stage 3 Arrangement of microorganisms with greater protection from predators, toxicants and environmental alterations, making it easier to obtain the nutrients necessary for the attachment of other microorganisms Stage 4 Increase in the capture of more particles and organisms, such as the larvae of marine macroorganisms, as a consequence of the pre-existence of the biolm and the roughness created by the irregular microbial colonies that comprise it Attached organisms Nature of lm formed Approximate initiation time 1 min

Adhesion of organic molecules, such as proteins, polysaccharides and proteoglicans and, possibly, some inorganic molecules Bacteria, such as Pseudomonas putrefaciens and Vibrio alginofyticos and diatoms (single-cell algae) such as Achnantes brevipes, Amphora coffeaeformis, Amphiprora paludosa, Nifzschia pusilla and Licmophora abbreviata Spores of microalgae, such as Ulothrix zonata and Enteromorpha intestinalis, and protozoans, including Vaginicola sp., Zoolhamnium sp. and Vorticella sp.

Conditioner

Microbial biolm

124 h

Biolm

1 week

Larvae of macroorganisms, such as Balanus amphitrite (Crustacea), Laomedia exuosa (Coelenterata), Electra crustulenta (Briozoa), Spirorbis borealis (Polychaeta), Mytilus edulis (Mollusca) and Styela coriacea (Tunicata)

Film consisting of the attachment and development of marine invertebrates and growth of macroalgae (seaweed)

23 weeks

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Table 3 Characteristics of main marine macroorganism species

E. Almeida et al. / Progress in Organic Coatings 59 (2007) 220 Table 4 Antifouling products used in the past, prior to mid 19th century Civilisation/navigator Oldest Phoenicians, Carthaginians Phoenicians Greeks Romans, Greeks Vikings Plutarch Several Columbus Various Approximate period Oldest 700 b.c. 700 b.c. 500 b.c. 300 b.c. 200 b.c. to 45 a.d. 10 a.d. 45125 a.d. 13th15th centuries 16181625 Antifouling product Wax, tar and asphalt Pitch and possibly copper sheathing Lead sheathing and tallow Coatings of arsenic and sulphur mixed with oil Wax, tar and lead sheathing Lead sheathing with copper nails Seal tar Scrapings of algae, slime and pitch Pitch and mixtures with oils, resin or tallow Pitch and tallow Copper, possibly with a mixture of cement, iron dust and a copper compound (sulphide) or arsenic ore Sacricial wood sheathing on a layer of pitch and animal hair Wood sheathing covered with mixtures of tar, fat, sulphur and pitch, with numerous metallic nails arranged with their heads forming a type of metallic sheathing Copper sheathing which was abandoned for causing galvanic corrosion of iron nails (To be dened) Copper sheathing, using nails of copper and zinc alloy Sir Humphrey Davy, after studying the copper corrosion process, demonstrated that copper dissolution in sea water prevented fouling Suggested sheathing of zinc, lead, nickel, arsenic, galvanised steel and alloys of antimony, zinc and tin, followed by copper-plated wood sheathing. Suggested non-metallic sheathing (rubber, ebonite, cork, enamel, etc.) Wood sheathing covered with copper sheathing (abandoned due to cost) Paints containing a toxicant (Cu, As or mercury oxide) dispersed in a polymeric binder (linseed oil, shellac, colophony) Hull type Wood

7

Reference [37,38] [37,38] [39] [38] [38] [37,38] [37] [37] [37] [37] [37,39]

18th century

[39] [37]

English (Frigate HMS Alarm) Various English English

1758

[37] [39] [39] Wood or steel [37,38]

1786 Early 19th century

Various

17581816

[38]

1862 Various Mid 19th century

[37] [37,38]

3.3. Antifouling technologies 3.3.1. First technologies used prior to mid 19th century The need to protect ship hulls from marine fouling is as old as mans use of ships as a means of locomotion. As can be seen in Table 4, which summarises the main antifouling products used prior to the mid 19th century, since ancient times use was made of natural products such as wax, tar and asphalt [37,38]. According to the same sources, the Phoenicians and Carthaginians seem to have been the rst to use copper for this purpose. This technique was similarly adopted by the Greeks and Romans, who also investigated the use of lead sheathing. In the 18th century it was common to use wooden sheathing covered with mixtures of tar, fat and pitch and studded with numerous metal nails, whose heads, closely in contact with each other, seem to have formed a sort of second metallic sheath [37]. Even in the 18th century several countries returned to the use of copper sheathing, with copper and zinc nails, and experimented with sheathings of zinc, lead, nickel, galvanised steel and other materials, as well

as copper-coated wood sheathing. Non-metallic sheathings were also suggested, namely of rubber, vulcanite, cork and others, which were eventually abandoned due to their high cost and/or difcult application. According to a recent publication, copper sheathing was abandoned with the British frigate Alarm, after the Second World War, when it was discovered that the iron nails used had corroded due to the galvanic action of the copper sheathing, with the consequent structural risks that this implied [37]. According to the same reference, in 1782 several French and British vessels sank on the coast of Newfoundland, causing close to 3500 deaths, as a consequence of such galvanic corrosion. These problems seem to have been overcome in 1786 when nails of a copperzinc alloy started to be used, which were sufciently resistant to be employed in shipbuilding. It was in this context that, after several attempts, and using lead and coated wood sheathings, the rst antifouling paints appeared in the mid 19th century, containing copper, arsenic or mercury oxide as toxicants dispersed in linseed oil, shellac or rosin [3739].

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Copper, arsenic or mercury oxides

Copper, arsenic or mercury oxides

Red mercury oxide or zinc oxide, zinc dust and India red Copper or mercury oxides

Copper compounds Copper compounds + co-biocides

Copper oxide Different toxicants

Pigment/biocide

Copper sulphate Copper compound

3.3.2. First antifouling paints used on steel hulls prior to 1960 The rst antifouling paints emerged in the mid 19th century and were based on the idea of dispersing a powerful toxicant in a polymeric binder. These were followed by other paints with binders based on different bituminous products and natural resins, whose dilution was achieved with turpentine spirit, benzene or naphtha [37,38]. However, since the pigments used in these paints, which were applied in direct contact with the ship hull, caused corrosion on the rst steel hulls, the application of a primer capable of protecting the hull was quick to appear. Meanwhile new products were emerging, including hot plastic paints with natural binders and copper or other toxicants, rust preventive compounds which were shellac-based products containing toxicants, and, with the development of polymer chemistry, cold plastic paints which used different synthetic resins or natural products alone or in mixtures [42]. The latter, which were easier to apply by means of airless spraying, which was also developed around that time, allowed dry docking intervals of up to 18 months. Table 5 summarises the main types of antifouling paints used on steel hulls prior to 1960. The rst organometallic paints (with tin, arsenic, mercury and others) [38] appeared around 1950 and gave rise, after numerous and successive developments [4349], to tributyltin (TBT)-based antifouling paints, which became famous due to their great efciency and versatility [1113]. 3.3.3. Main types of products used on steel hulls in the second half of the 20th century These paint products, systematically based on the dispersion of toxicants in different types of polymeric binders, have become differentiated over recent decades according to the mechanisms they use to release the toxicants in the sea water. These mechanisms determine the application, behaviour and duration of the antifouling coatings obtained. Table 6 summarises the main types of antifouling paints in this group, schematising their behaviour mechanisms and the release rate of their toxicants over time. 3.3.3.1. Toxic pigments. As shown in Table 4, man has long been familiar with coppers properties as a toxicant to a large number of marine organisms. Consequently, all traditional antifouling paint types use copper compounds as their pigments. The way in which copper compounds are released is well known, being described by the following reactions [50]:1 + 2 CuO(s) + H 1 + 2Cl 2CuCl2 + 2 H2 O

[37,38,41,42]

[37,38]

[40]

With alcohol, turpentine essence or pine tar oil Easier to apply than hot plastic paints Some allowed dry dock intervals of up to 18 months and application by airless spraying Coal tar or coal tar + colophony Shellac varnish Synthetic resins 1926 19081926

Dispersion of toxicant in polymeric binder Insulation of hull from antifouling paint by application of varnish Similar to Moravian Italian Moravian

Linseed oil, Shellac varnish, tar, resins Idem, with preliminary insulating varnish coating Metallic soap composition Colophony

Tar Shellac primer and Shellac antifouling paint Grade A Gum Shellac

Main components

Table 5 Types of antifouling paint used on steel hulls prior to 1960

Binder

Mid 19th century

1863 Late 19th century

First used

1847

1860

19501960

Acrylic esters or others

Some seemed capable of resolving the problem of marine fouling reasonably well

With naphtha or benzene Shellac type paints

Remarks

[40,4349]

Reference

[40]

[37] [37]

[37]

(1) (2)

Application of insulating primer under antifouling paint Hot plastic paints

CuCl2 + Cl CuCl3 2First organo-metallic paints

in which reaction (1) is irreversible and is inuenced by the kinetics, while reaction (2) is instantaneous and reversible, and may be considered to be in permanent equilibrium. Since sea water is an oxygenated medium, copper complexes are quickly oxidised to Cu2+ , which is the main biocidal form, originated by cuprous oxide [50]. However, copper ions are not equally efcient against all types of fouling organisms, and the latters sensitivity to copper

Type of product

First paints

Cold plastic paints

Spirit varnish paints

Antifouling paint Rust preventer

Table 6 Main types of antifouling paint used on steel hulls in the second half of the 20th century Type of paint Since Main components Binder Pigment/biocide Proposed mechanisms (courtesy of Hempel Portugal) Toxicant release in time (courtesy of Hempel Portugal)

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Soluble matrix

1950

Colophony and others Copper, arsenic, zinc, mercury or iron oxides

Insoluble matrix or contact paint

1955

Acrylic resins, vinyl resins or chlorinated rubber polymers

Copper and zinc oxides with or without organo-metallic compounds

Self-polishing paints containing tin (TBT-SPC)

19741985

Acrylic polymer Zinc oxide and insoluble (normally methyl pigments or copper oxide, meta-acrylate) with tri-organo-tin and co-biocides TBT groups bonded to main chain by ester binders (copolymer)

9

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ions decreases in the following order: microorganisms, invertebrates, algae, bivalves and macrophytes [51]. Furthermore, the existence in the aquatic environment of water-soluble chemical ligands which can become bonded to copper ions signicantly reduces their toxicity by reducing the concentration of copper ions available as biocides [51]. The same reaction is performed by some copper chelators, which are synthesised and excreted by the microorganisms themselves, as well as the natural and spontaneous formation of green malachite (CuCO3 Cu(OH)2 ), which is slightly soluble [52]. Apart from copper oxides, use is at times made of zinc(II), iron(III) and titanium(IV) oxides, and, in colour paints, copper thiocyanate, in view of their good solubility characteristics [53,54]. The latter produces CuCl2 concentrations similar to copper oxide in the conditions existing in sea water. Among the inorganic products used as biocides, the biocidal characteristics of the aforementioned soluble pigments of zinc and iron oxides have not yet been fully claried [31]. Numerous organometallic compounds have also been studied as biocides, among which special mention should be made of RMeX type compounds, where R is an organic radical, Me is a metal and X is a halide or an acid residue. Among these, reference may be made to organomercury (RHgX), organoarsenic (RAsX), organolead (RPbX), and more recently organotin compounds (RSnX). Among the latter, attention is drawn to tributyltin (TBT) and its derivatives, which are highly toxic to oysters, molluscs and crustaceans [48,49]. These products are successively being banned for environmental and bioaccumulation reasons. Although copper and TBT and their derivatives present wide ranging protection against marine organism fouling, they have often been used in conjunction because microalgae and amphora are tolerant to copper while brown and green algae and certain diatoms are tolerant to TBT [55]. However, with the boom in the use of TBT-based paints, due to their efciency and versatility, it was found that they have disastrous effects on the marine environment in the vicinity of dry docks and busy ports. These include their accumulation in mammals and the weakening of sh immunological systems. Consequently in October 2001, when the IMO took stock of the adverse effects of TBT on the marine environment, an order was issued banning the use of this type of biocides in the manufacturing of antifouling paints as from 1st January 2003, and the presence of these paints on ship surfaces as from 1st January 2008 [10]. Similar measures were not implemented for copper, since this is an essential element that is required for the normal growth of all plants and animals, and thus is naturally present in sea water. It has even been estimated [56] that the amount of copper released by antifouling paints barely amounts to 3000 tonnes per year, an insignicant amount compared to the 250,000 tonnes originating from natural sources. On the other hand, copper is lipophilic and only shows a slight tendency towards bioaccumulation [51], which explains why it has remained in the formulations of antifouling paints over the years. Therefore, even though there are some doubts concerning the action of high copper concentrations in sea water on certain marine organisms [47,5759], it is normally considered that the low bioavailability of copper ions released by antifouling

paints seems to present an environmental prole that complies with current environmental quality standards. Meanwhile, as it is indispensable to complement the toxic action of copper, and TBT has been banned from antifouling paints, other biocides have been developed and used to ll this gap [60]. Among the reinforcing biocides (cobiocides) currently available, reference may be made to Irgarol 1051 (2-methylthio-4-tert-butylamine-6-cyclopropylamine-striazine), Diuron (3-(3,4-dichlorophenyl)-1,1-dimethlyurea) [61], copper pyrithione [62], zinc pyrithione [63], Sea-nineTM 211 (member of 3(2H)-isothiazolone) [62], and Zineb [64,65]. Although their effects have not been fully studied, according to Voulvoulis et al. [65] zinc pyrithione and Zineb seem to be the least harmful to the environment, while Irgarol and Diuron are seen to be more harmful. More recently, and somehow taking up an idea implemented empirically between a.d. 45 and 125 (see Table 4), in which the slime lm and scrapings of algae were mixed with pitch to protect ship hulls [37], studies are being made of the biocidal action of the fouling organisms themselves. In these studies it has been seen that secondary metabolites of these organisms may repel or prevent their attachment to ship hulls [6675]. This is an issue that continues to be under study at the present time, as will be mentioned below. 3.3.3.2. Soluble matrix paints. These paints, with binders based on rosins and their derivatives [25], incorporate toxic pigments such as copper, iron or zinc oxides, and previously also arsenic and mercury. They started to be developed in the 1950s and are soluble in sea water, present poor mechanical strength, and only allow the inclusion of low concentrations of biosoluble materials and the application of relatively ne lms [7679]. Therefore, in view of the constant erosion that they suffer in service [80], they do not assure protection for more than 1215 months (see Table 6). Their main advantage is that they can be applied on smooth bituminous-based primers. Their main disadvantages are related with the sensitivity of the binders to oxidation and oil pollution. This means that ship hulls coated with these paints need to be reoated as soon as possible after dry docking, in order to avoid oxidation in contact with the atmosphere. Furthermore, their relatively weak biocidal activity in stationary conditions makes them unsuitable for slow speed vessels or ships that remain idle for long periods [31]. 3.3.3.3. Insoluble matrix paints or contact paints. This type of antifouling paint uses high molecular mass binders, such as acrylics, vinyls or chlorinated rubber, all of which are insoluble in sea water. In view of their good mechanical strength characteristics (for which they are also known as hard antifouling paints) they can incorporate high toxicant amounts, whose particles can be in direct contact with each other and consequently be released gradually. Since the binder is not soluble in sea water, as the toxicant agents it contains are released, the sea water spreads through the pores that are left empty by the latter and go on to dissolve the next toxicant particles (see Table 6). However, as the exposed toxicant particles are deeper in the

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paint lm, the toxicant release rate gradually decreases in time, and the protection afforded becomes increasingly less efcient [81]. For its part, the honeycomb structure left in the coating contributes to making the surface rougher and more liable to retain pollutants from the sea water, a fact that also contributes to blocking the release of toxicants. For these reasons the duration of the coatings obtained with these paints is between 12 and 24 months, depending on the severity of the exposure conditions, which limits their application on some types of ships [82]. Given the high inertia of the polymers used as binders in this type of paint, the ships protected with them experience fewer dry dock problems, thanks to their oxidation and photooxidation stability [81]. 3.3.3.4. TBT self-polishing paints. Compatible with both steel and aluminium hulls, these paints are based on an acrylic copolymer (normally methyl methacrylate) with TBT groups bonded to the main polymer chain by ester bonds [83,84], in which the polymer is soluble in sea water. Since this dissolution can be controlled at molecular level, it is possible to obtain a well-known self-polishing effect in these paints. Besides the toxicants that react inside the copolymer, these paints include toxicant pigments, such as copper oxide, and thus present highly efcient antifouling properties in any service conditions at sea. After immersion, the soluble pigment particles come into contact with the sea water and start to dissolve. Unlike insoluble matrix paints, in this type of paint, where the TBT methacrylate and methyl methacrylate copolymer is hydrophobic, the water is prevented from penetrating the lm [64]. Thus, the sea water barely manages to ll the pores created by the dissolution of the soluble pigment particles, as represented in Table 6. As the carboxyl-TBT bond is hydrolytically unstable in slightly alkaline conditions, like those found in sea water, slow and controlled hydrolysis of the coating takes place, which corresponds to the wear of the polymer, according to a reaction conned to a few nanometres from the surface [83], which may be represented by Eq. (3):

contrast to soluble or insoluble matrix paints, or to apply a sealer or tie coat during recoating operations [1152]. Other characteristics of these paints have been described in a recent publication by Omae [52]. However, it should be noted that their binder composition is so controlled that it allows the formulation of the most suitable antifouling paints for any type of ship. Thus, for fast vessels, which are especially sensitive to increases in fuel consumption, and thus require much more efcient antifouling protection [17], products with a low polishing rate have been formulated, while for slow vessels or those that spend long periods in port, products with a faster polishing rate have been formulated, in order to assure the most suitable rate of release for the adequate control of marine fouling [16]. The importance of these paints is such that in 1999 it was estimated that close to 70% of all commercial shipping was protected by them, achieving direct savings of close to US$ 2400 million a year in fuel and other costs [57]. 3.3.4. Alternatives to painting In parallel with the development of antifouling paints, over the years a number of alternative techniques to painting have been developed, many of which were systematised by Swain, who grouped them under the headings of radiation, chemical, electrical, surface treatment and thermal techniques [88]. In the 1950s ultrasonic vibration techniques were tested for this purpose [89], but were found to be insufciently efcient since they were too expensive to be applied on ships. The use of radioisotopes, such as 60Co, 204Tl and 91Y, prevented the xing of marine fouling but only at intensities (ca. 20 rad/h) that made this technique excessively hazardous; at lower intensities they did not prove to be sufciently efcient [90]. The pumping of solutions of toxicant agents in petroleum over ship hulls did not show satisfactory antifouling efciency, besides having a high economic and environmental cost [89]. The use of Teon [91] and silicon rubber [92] nishes, also tested as antifouling methods, while only allowing the temporary

(3) In time, sea water dissolves more pigment particles, causing the releasable area to grow and making the copolymer lm brittle and easily erodible by sea water, leaving a new fresh area of the coating uncovered for subsequent release (self-polishing effect). Paints of this type are normally formulated to have a polishing rate of close to 520 m a year [80], which has allowed dry docking intervals to be extended up to periods of 5 years [11,8587]. As may be observed in Table 6, the release rate of these paints is approximately constant [26], and they are active regardless of the ship sailing speed. Other further advantages of this type of paint stem from the fact that it is not necessary to remove their porous residues, in xing of algae and barnacles [93,94] (whose adhesion to ship hulls was sufciently weak to allow their easy removal by water jets or simply by the friction of sea water in the case of fast ships), was abandoned due to failure to overcome the problem of lack of adhesion of these materials to ship hulls at that time [91,92]. Different electrical techniques have also been tested, among which mention may be made to the use of electrical currents to prevent the attachment of marine organisms, which more recently led to the use of impressed currents as a cathodic protection technique for ship hulls [95]. The use of electrical

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pulses or direct current is not only relatively expensive but also originates deposits and rough surfaces [89]. In other electrical methods use was made of a conductive paint coating that acted as the anode in the electrolysis of sea water, giving rise to the release of chlorine which then served as a biocide [96]. The paint used consisted of a graphite and coal suspension (9:1 parts by weight) in a plastied chlorinated rubber binder. For the release of chlorine it was sufcient to apply 1 V at a current density of 10 A cm2 , which originated the antifouling effect of the technique. Other electrical techniques involved the formation of ozone bubble curtains [97,98], copper ions [99], platinum compounds [100,101], ammonia [102], bromine compounds [103], or oxygenated water [104], generated electrolytically. Kerr et al. [104], using very low surface potentials (66 mV versus SCE) managed to reduce the bacterial population on a reference sample by 12%, but this reduction is insufcient for application on ships. With the aim of developing more environmentally friendly systems, techniques have been used based on the direct transfer of electrons between an electrode and the microbial cells themselves, giving rise to the electrochemical oxidation of the intracellular substance. In order to avoid the use of high potentials, Okochi and Matsunga [105] proposed the use of ferrocene derivatives as redox mediators to avoid the release of chlorine. Using the same principle Nakasono et al. [106] resorted to the use of chloroprene carbon sheet. Other trials considered the use of cladding with copper alloys or metallic coatings of zinc or cadmium and their alloys, the latter applied by ame spraying, in which toxicants such as arsenic and cadmium compounds were anodically dissolved [106]. By applying a potential of 1050 mV relative to the silver/silver chloride electrode for zinc coatings, or 750 mV for cadmium coatings, the metallic coating was dissolved, releasing the zinc, cadmium or arsenic poisons. The amount released was a function of the current density, which was of the order of 10 A cm2 . Taking special care, according to the type of ship in question, these techniques made it possible not only to prevent the attachment of marine organisms on their hulls but also to protect them from corrosion. However, prohibition of the use of arsenic, cadmium and other toxicants, along with the relatively complex maintenance of these techniques and/or their high cost, led to the abandonment of their use. Using inorganic coatings but not metallic coatings, Matsunaga and Lim [100] and Nakaiama et al. [107,108] have more recently but with the same objective used plates coated with titanium nitride obtained by physical arc spraying deposition. They showed that the application of potentials of 1 and 0.6 V relative to the silver/silver chloride electrode inhibited the xing of organisms on these plates. With the same aim, Wang et al. [109] investigated polyaniline coatings containing toxicants, showing that although a synergetic effect could be seen between the polyaniline and the toxicants, the antifouling effect observed was relatively poor. Among other attempts to obtain antifouling effects, mention may be made of the use of piezoelectric coatings [110], different types of radiation, such as acoustic and ultraviolet, magnetic elds, and even thermal actions, from heat to cryogenics [97].

However, although many highly differentiated antifouling alternatives have been studied (always with a limited number of marine organisms, and very especially with bacteria), the difculty and/or cost of their application on ship hulls has limited the extension of their study to other species of marine organisms found in the different seas and oceans of the world [20,21,111114]. Nevertheless, the development of some of them has led to more restricted specic applications, such as in oyster beds and medical applications [31]. 4. More environment- and man-friendly antifouling paints Due to the environmentally harmful action of the well known, efcient and versatile TBT self-polishing paints, and the consequent prohibition of their application after 1 January 2003 and of their presence on ship surfaces after 1 January 2008 [9,10], paint manufacturers have been forced to urgently study and develop new more environmentally friendly antifouling paints. Without seeking to be complete or exhaustive, the products with biocides that have recently been marketed for this purpose may be grouped under three main headings, namely controlled depletion paints (CDPs), tin-free self-polishing paints (TF-SPCs) and hybrid systems. Table 7 summarises the main alternatives developed recently, with the inclusion of copper oxide and cobiocides. Meanwhile, due to ecological pressures on the one hand, and, on the other hand, to the fact that these paints, while free of TBT, are always based on the release of biocides and co-biocides, whose action has not always been fully claried, in recent years there is a tendency towards the development of fully biocide-free antifouling paints. 4.1. Tin-free controlled depletion paints (CDPs) These paints, which constitute the rst generation of tin-free antifouling paints, are no more than a development of traditional soluble matrix paints, whose binder is reinforced by organically synthetized resins which are more resistant than rosins and control the dissolution of the soluble binder. However, their working mechanisms are assumed to be similar to those of conventional rosin-based paints. Also known as ablative/erodible paints, they contain a large proportion of physically drying, non-toxic, sea watersoluble binder, combined with polymeric ingredients capable of controlling the relative rate of the dissolution/erosion mechanisms by means of physical processes. The biocide load integrated in them may be regulated above the level presented by a good non-self-polishing paint. In contact with sea water, the biocides dissolve together with the soluble binder, and the dissolution process-controlling ingredients are washed from the surface. The constant ablation/erosion rate seems to be achieved through the equilibrium of the process, which is attained a short time after immersion (Fig. 3).

Table 7 Main alternatives developed or under development in the last decade Type of paint Controlled depletion paints (CDPs) (increase of soluble matrix technology with use of new resins) Company Chugoku Marine Paints Kansai Paint International Marine Coatings Transocean M.P.A. Tin-free self-polishing systems (TF-SPCs) (identical mechanism to self-polishing technology but without tin) Ameron Chugoku Product designation TFA-10/30 Sea Tender 10/12/15 New Crest Interspeed 340 Interclean 245 Optima 2.30-2.36 ABC-1a4 Sea Granprix 500/700 (2nd generation) Sea Granprix 1000/2000 (3rd generation) Globic Series Zinc or copper acrylate-based binder integrating biocides and others Silil-acrylate-based binder integrating biocides and others Synthetic colophony substitutes with reduction of co-binder (plasticizer) reinforced with mineral bres and product 8190081970 as potential biocide, or sea.nine/Cu pyrithione Basic known components Proposed mechanisms CDP. Hydration CDP. Hydration CDP. Hydration CDP Contact leaching CDP SP. Hydrolysis SP. Zinc acrylate. Hydrolysis E. Almeida et al. / Progress in Organic Coatings 59 (2007) 220 [102104] Reference

SP. Silil-acrylate. Hydrolysis SP. Ion exchange. Fibres RCOOZnOOCR + 2Na+ 2RCOO Na+ (s) + Zn2+ (aq)

[102104] [8894]

Hempel

International Marine Coatings and Nippon Paint Jotun

Oceanic Series Olympic Series Intersmooth Ecoex SPC

Kansai Paint Sigma Coatings Hybrid systems (CDPs/SPCs) International Marine Coatings Hempel

Sea Quantum (Plus, Classic, Ultra, FB) SeaQueen SeaPrince SeaGuardian Exion Nu Trim Alphagen 10-20-50 Sigmaplane Ecol (also HA) Interswift 655 Combic Series

Acrylic matrix bonded to copper salts of an unrevealed organic chain. Potential biocide zinc pyrithione Potential biocide Cu pyrithione

SP. Ion exchange. Fibres SP. Ion exchange. Fibres PCOOCuOOCR (s) + Na+ PCOO Na+ (s) + RCOO Na+ (aq) + basic copper carbonate SP. Silil-acrylate. Hydrolysis SP. Copolymer binder SP. Copolymer binder SP. Copolymer binder SP. Zinc acrylate. Ion exchange SP. Hydrolysis SP. Hydrolysis and ion exchange SP. Hydrodissolution Hybrid of CDP + SPC SP + saponication

[8894]

Potential biocide isothiazolone Potential biocide isothiazolone Potential biocide Zineb or Cu pyrithione

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Fig. 3. Proposed mechanism for ablative/erodible paints (courtesy of Hempel Portugal).

These tin-free products are not ideally eroded like TBT-based self-polishing paints. They give rise to an empty matrix which will affect their mid-term behaviour. However, this hollow matrix may be removed before repainting. Due to their action mechanism they need high copper and co-biocide contents, with the environmental disadvantages that this implies [16]. Among the general properties of ablative/erodible paints, mention may be made of the longer than 3 years protection period that they can afford, their toxicant release economy, roughness control, and the fact that they do not need a tie coat during repainting in the dry dock, besides being less expensive than TBT-based self-polishing paints. They are widely used on leisure boats and small ships with relatively short service times [17].

4.2. Tin-free biocide-containing self-polishing paints (TF-SPCs) In this type of paint the products are integrated in an acrylic matrix to which different pendent groups of the main chain are added, however without tin. Like in self-polishing paints containing tin, the pendent groups are considered to be released in contact with sea water (Fig. 4). Nevertheless, and despite the high number of patents registered in this domain until 1996, these groups are in no case as effective as TBT [31]. This is because of the signicant impact of the chemical nature of the pendent groups on the balance of the hydrophilic/hydrophobic characteristics of the matrix, the alteration of the vitreous transition temperature during hydrolysis, the absorption of water, and

Fig. 4. Tin-free antifouling mechanisms involving different pendent groups (courtesy of Hempel Portugal).

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These polymers interact with sea water, and their selfpolishing effect is seen with the controlled release of biocides [17]. Due to their relatively high polishing rate, the maximum service life of this type of paint is normally around 3 years, although in some cases service lives of 5 years have been reported [9,116,117]. However, according to various authors, they do not achieve the same level of efciency as TBT-based self-polishing paints. 4.3. Hybrid paints If the knowledge of the mechanisms by which CDPs and TFSPCs act continues to be limited and practical experience with them is still relatively short, the situation is much more complex when it comes to clarifying the mechanisms by which hybrid paints act. These paints, which are starting to be marketed, are based on the simultaneous action of different mechanisms and at times incorporate new developments, as in the case of the microbres introduced by Hempel. 4.4. Biocide-free paints Once marine organisms segregate polar uids that function as adhesives on immersed surfaces, one way to stop them from becoming attached to these surfaces, without using biocides, seems to be by the development of low energy non-polar coatings, such as those previously tested and reported by de la Court and Vries [89,91,92], but which, at that time, were abandoned due to their lack of adhesion to steel hulls. However, although it is very difcult to develop biocide-free antifouling paints that are efcient at an acceptable price, in the current environmental climate the pressures to use environmentally friendly products have led to the development of almost innocuous products, from the point of view of environmental hazard. These products are totally different than traditional antifouling paints, acting essentially by means of a barrier layer

Fig. 5. Mechanism proposed by Hempel for a TF-SPC paint (courtesy of Hempel Portugal).

the possible intumescence of the polymer, among other reasons [115]. Furthermore, these paints are free of rosins and their derivatives, which means they possess better photostability, an important characteristic for the boottop area. They are generally based on copper acrylates combined with co-biocides, but use is also made of zinc acrylates and others. The antifouling products in this group use various different technologies, and Figs. 5 and 6 offer examples of the mechanisms proposed by Hempel Portugal and by Chugoku, respectively, for different types of TF-SPCs. The rst case (Fig. 5) shows an ion exchange mechanism between a water-soluble zinc carboxyl polymer salt and the sodium ions in sea water. The second case (Fig. 6) shows a superactivated hydrolysis mechanism, including different steps of renewal, retention and release.

Fig. 6. Mechanism proposed by Chugoku, for a TF-SPC paint (courtesy of Chugoku).

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and at the same time possessing ultra-smooth surfaces which assure very low friction and are hydrophobic, so that marine organisms cannot adhere to them [82,118,119]. Any transient fouling will be removed by the movement of the sea water, in a self-cleaning process, or by the action of a simple pressurised water jet [120]. According to Brady [121], these products will lead to surfaces that are capable of possessing: (i) A signicant number of active groups that can move freely over the surface, conferring it the desired surface energy range; (ii) A main chain of a linear structure which prevents undesirable interactions; (iii) High molecular mobility of each chain and of the active lateral chains; (iv) Low modulus of elasticity; (v) A smoothness at molecular level which prevents the inltration of biological adhesives, and therefore their xing; (vi) A thickness that can control interface fracture mechanisms; (vii) Absence of polarity; and (viii) Molecules that permit all the above requirements and at the same time are stable for long periods in sea water. Of all the polymers currently available (v. [120] ranking of the resistance to barnacle adhesion of different polymers determined according to ASTM standard [121]), only two groups seem to full these requirements, namely uoropolymers and silicones. However, even though uoropolymers originate very low energy non-porous surfaces with good anti-adhesive characteristics, the presence of uorine atoms in their structure refrain their rotation close to the chain bond, limiting the required surface mobility [123]. Probably for this reason, the use of polyurethane-based and epoxy-uorinated antifouling products has not proven to be efcient [96]. If silicones are applied in relatively thick layers they present signicantly superior anti-adhesive characteristics to uoropolymers [124]. In view of their low surface energy, low microroughness and low vitreous transition temperature [96], polymers based on poly(dimethylsiloxane) are currently used in the formulation of antifouling paints. The coating surfaces obtained with these products seem to present sufcient mobility to discourage the functional groups of numerous marine organisms from adhering to them [123]. However, after 3 years of exposure in sea water these completely biocide-free antifouling paints are barely able to prevent the attachment of marine organisms on around 20% of the exposed surface, for which reason they can only be efciently applied on high speed vessels (22 knots). This, because, at this speed sea water can remove the weakly attached fouling on these surfaces [124]. These products can also be used on vessels that sail at speeds of between 15 and 30 knots, provided they are highly active and are idle only for short periodes. According to Ryle [125], the minimum speed for the attachment of barnacles is 7 knots, while the speed necessary to remove algae is 18, or 30 knots for slime lms.

Antifouling products based on poly(dimethylsiloxane) are normally applied on a primer coating and a sealer or tie coat. The primer must an assure strong adhesion to the ship hull, while the tie coat must assure compatibility/adhesion between the primer and the antifouling product. These systems have been shown to allow less barnacle xing than TF-SPC paints [126]. Their disadvantage is that they are relatively expensive (two to seven times more expensive than TF-SPC paints [126]), can cause silicon contamination problems, and are sensitive to mechanical damage, making it necessary to use suitable application and cleaning technologies [120]. 4.5. Some recent or current developments Although animal hairs have been used in the past in the preparation of primitive anticorrosive products (see Table 1), in the 1990s an interesting innovation was made in antifouling paints, with the introduction of ne bres in their formulation. This technology was initially based on the use of relatively short bres (e.g. 1.0 and 1.3 mm in length) in a dense prole (close to 200 bres/mm2 ). After the application of an epoxy adhesive, the bres were electrostatically charged and applied by spraying, in order to assure their orientation perpendicular to the surface before the drying of the adhesive. When the coating was submerged, the bres moved with the action of the current, giving rise to a movement on the coating surface which prevented the attachment of marine organisms [126]. By the turn of the 21st century, a number of paints integrating synthetic microbres have appeared on to the market (e.g. 50100 m in length and 210 m average thickness [31]) with the aim of improving the efciency of many binders such as methacrylates [127], acrylates, silylates and others [31]. In combination with the binder, the bres form a three-dimensional structure which originates an extremely strong and exible coating, while at the same time maintaining the characteristic smoothness of a self-polishing antifouling paint [86]. These bres allow the use of a large amount of binder in the paints, resulting in the highest solids content of all current self-polishing paints, with good control of the polishing process. These products are available in different polishing rates and can, in some cases, be specied for 35 years of service on all underwater ship parts, depending on their formulation and the hull type in question [86]. However, the increase in hull roughness can contribute to a certain drag effect, and it has recently been veried that shorter bres yield better results than longer bres [127]. Meanwhile, according to Abarzua and Jakubowsky [67], the secondary metabolites of some marine organisms seem to be able to act enzymatically through the dissolution of adhesives. By interfering with the metabolites of marine organisms, inhibiting their xing and their metamorphosis and growth, these metabolites can act as natural biocides [68,69]. This type of application seems to have been experimented empirically during the times of Plutarch (a.d. 45125), when, as can be seen in Table 4, scrapings of algae and slime were mixed with pitch as an antifouling method. Among these secondary metabolites, organic acids, steroids, terpenoids, amino acids, alkaloids,

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polyphenols, acetogenins and heterocyclics such as furans and lactones have been identied [7375]. Among the compounds identied, the best ecotoxicological prole is presented by zosteric acid, which is a sulphoxy-phenolic acid derived from marine zostera, which inhibits the accumulation of marine organisms by interfering with their adhesion [18]. However, although promising, no reliable conclusions can be drown from the current state of development of enzymatic natural biocides, although it is hoped that they may lead in the near future to enzymatic antifouling systems of a certain efciency. Active antifouling agents have already been identied in bacteria [128,129], algae [130,131], corals [132], sponges [133135], seaweed [136] and land plants [137,138]. Studies are now under way into the mechanisms by which they can be combined with the polymeric matrices of antifouling paints in order to allow the development, on the coating surfaces obtained with them, of a sufcient action to prevent the attachment of marine organisms, without leaching out the compounds and without contaminating the environment. Another eld of antifouling currently under research stems from the observation and study of the surface of certain marine animals, such as dolphins and killer whales, who spend their entire lives at sea without suffering signicant incrustations on their skin, which clearly has anti-adhesive properties. According to Swain [97], this seems to be due to the presence of low surface energy glycoproteins in their skins. Meanwhile, a study performed by Baum et al. on the skins of killer whales [137] revealed a hydrated lm of a nanoroughness structure with a gel, characterised by a prole of pores with nanocrests, the pore size which is smaller than the average value of the skins of most marine animals. This lm, of great elasticity and energy dissipation capacity, is also rich in hydrolytic enzymes. From a hydrodynamic point of view, the three groups of marine animals of greatest interest in this eld of research are cetaceans (whales and dolphins), teleosts (bony sh) and eslamobranchs (sharks, rays, etc.) [139,140]. Meanwhile, and though still at an early stage, all the observations that are being made in these elds seem to suggest the use of microstructured silicones, like those proposed by Andersson et al. [140], to prevent the attachment of marine organisms. However, the latters efciency has been seen to be modest in relatively cold waters, for which reason their efciency in warmer regions is probably insufcient. Multilayer systems have also been proposed. The topcoat layer may be periodically removed, but there is no news of real practical development in relation to this type of application [120]. In another line of research, Hoffman [141] and Galaev [142] suggest the introduction of multiple associated response stimuli in the so-called intelligent polymers. These authors claim that certain substances can control the permeation rate of a polymeric matrix when its pores are coated by them. The question is thus posed as to whether it will 1 day be possible to create a coating that is capable of selectively releasing certain substances in response to articial stimuli (electricity, sound, etc.) or natural stimuli (pH, water temperature, nature of fouling adhesives or others) or, in other words, whether it will 1 day be possible to

obtain coatings that are activated only when marine organisms try to attach themselves or only during seasonal periods [31]. Results can be expected much sooner in research lines related with the underwater mechanical cleaning of fouled surfaces, which aim to avoid the need for the frequent drydock of ships for cleaning purposes [143]. The most important issue here is to make it possible to coat ship hulls with toxicant-free products which allow the marine organisms that foul these hulls to be easily removed or relatively so. In this case it will only be necessary to provide special protection in the areas that are difcult to clean by means of remote-controlled automatic cleaning vehicles. To increase their cleaning efciency, these automatic systems resort to the use of ultraviolet radiation, ultrasound, laser beams, etc. For this purpose research lines are under way which allow the application on different types of ships of robot cleaning equipment similar to that currently used by the Air Force. The potential cost of such underwater cleaning may even be cheaper than cleaning with high pressure water jets, which is the method currently carried out in the dry dock [143]. In turn, it seems that underwater cleaning may be used on recently developed biocide-free coatings without damaging them. Finally, there have also been certain developments in the laboratory methods used for the efcient assessment of new antifouling paints [12,122,126,144148]. 5. Conclusions Since remote times Man has been ghting a never-ending battle against the xing of marine organisms on surfaces immersed in sea water in general, and on ship hulls in particular. Even when the problem seemed to have been solved, thanks to the boom in the development of TBT-based antifouling paints, with their well-known technology in which, by suitably controlling the molecular composition of the binder, it was practically possible to tailor-make antifouling paints to meet the needs of each particular type of ship, it was soon to become an issue once again. This, because of the lipid-soluble nature of TBT, which means it is easily captured by cells, where it inhibits energy transfer processes in respiration or in photosynthesis which are indispensable for underwater life. Its harmful effect for marine organisms has meant that, in the vicinity of dry docks and busy ports, certain species have simply disappeared, such as Nucella lapillus, in which the females present male characteristics at TBT concentrations of less than 1 ng l1 . It was thus necessary to ban the use of TBT, at the initiative of several countries, the International Maritime Organization and the European Union, completely prohibiting its application after 1 January 2003 and its presence on ships after 1 January 2008. Meanwhile, the numerous alternative techniques to antifouling painting which have been tested over time have either not proven to be sufciently efcient or are so expensive and/or difcult to apply on ship hulls that they have not been applied with the hoped-for success. Thus, in a rst attempt to address the problem, antifouling paint manufacturers replaced TBT in self-polishing polymers with other chemical ligands of their main chains, such as copper, zinc or sylilated radicals, and reinforced the biocidal effect of

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E. Almeida et al. / Progress in Organic Coatings 59 (2007) 220

copper with articial biocides such as certain known herbicides and pesticides. However, many of the latter have also proven to be highly harmful to the environment, and the long term effect of many others has not yet been fully claried. Meanwhile, the implementation of the European REACH programme (Registration, Evaluation and Authorisation of Chemicals) and others imposes certain requirements for the acceptance and registration of new biocide products which encourage the abandonment of this type of products in sea water. In these conditions, antifouling paint manufacturers have no alternative but to intensify their research in the quest for biocidefree products that prevent the attachment of marine organisms, concentrating on the level of interfacial adhesion with coating surfaces or on achieving their removal by friction with sea water or washing with pressurised water jets. This is a highly ambitious objective, because although barnacles can be removed from surfaces to which they weakly adhere merely by simple friction with sea water, this is not the case with other marine organisms such as diatoms, algae, oysters and calcareous polychaetes, which become rmly xed to the surfaces and are not easily removed, even on high speed vessels. Thus, going back to ideas that have been tried out a different times in history (albeit in a totally empirical way), microbres have been introduced in recent formulations and a number of research lines are currently reassessing ideas such as the obtainment of new natural biocides from marine organisms themselves (rudimentarily practiced in the 18th century), novel anti-adherent polymers (already attempted in the 1970s), and new underwater cleaning techniques based on technologies already applied in the Air Force. In the rst of these cases, extracts have been taken from numerous marine organisms and studies are now under way into the mechanisms by which they could be combined with polymeric matrices to allow the development of surfaces capable of preventing the attachment of marine organisms without leaching out the compounds or contaminating the environment. Meanwhile, although promising, the current state of development of enzymatic-based natural biocides from secondary metabolites of different marine organisms does not yet allow reliable conclusions to be drawn, but it is hoped that enzymatic antifouling systems of a certain efciency may be obtained in the near future. In the second case, in view of their notable anti-adhesive surface characteristics, antifouling paints based on silicones, specically poly(dimethylsiloxane), have been developed and are now commercially available, having proven to be signicantly efcient for the protection of high speed ship hulls. Nevertheless, work continues on this line of research in the form of painstaking studies of the skin of marine animals such as killer whales, dolphins, bony sh and cartilaginous sh. The question is now being asked as to whether this line of research could lead to the development of intelligent antifouling paints which would be able to act when requested to do so, i.e. when marine organisms try to attach themselves to the surface or when ships are idle in port. In the third case, the development of new surface cleaning techniques using automatic robots and remote-controlled equipment could constitute an excellent contribution to lengthening

the service life of anti-adhesive paints applied on high speed ships, which could be cleaned more frequently at a more competitive price without the need for frequent dry docking. Meanwhile, and until such a time as more efcient completely biocide-free products are developed and marketed, the only environmentally acceptable alternative for application on other ships seems to be the old alternative of copper-based antifouling paints, despite their limited efciency. Acknowledgements The authors wish to thank Hempel Portugal and Chugoku for their contribution with the same gures used in this paper. References[1] [2] [3] [4] [5] [6] [7] [8] A. Milne, Shipbuiding Progress 33 (1986) 33. P.S.Z.N. Boero, Marine Ecol. 15 (1) (1993) 3. E. Almeida, Bol. Electr. Corros. 72 (1974) 19. I.S. Walker, PCE (1999) 24. Kerry Pianoforte, http://www.coatings world.com, May 2004, p. 32. M.A. Champ, Sci. Total Environ. 258 (2000) 21. A. Abbot, P.D.W. Arnold, A. Milne, Sci. Total Environ. 258 (2000) 5. U.S. Environmental Protection Agency, Uniform National Discharge Standards for Vessels of the Armed Forces, Final Rule, Federal Register 63, 176, September 1998, p. 11. Y. Honda, Technology after ban TBT in Japan, in: Proceedings of the Emerging Non-metallic Materials in Marine Environment, Honolulu, HI, USA, March, 1997, p. 18. IMO-MEPC 38 (1996) (v), Terms of reference for a corresponding group on the reduction of harmful effects of the use of antifouling paints for ships, IMO-MEPC Paper MEPC 38/WP.6, 1996. C.D. Anderson, IBS, UK, IBC UK Conferences Limited, UK, 1998, p. 1. S. Kiil, et al., Ind. Eng. Chem. Res. 40 (2001) 3906. U. Guerrik, U. Schneider, U. Stewen, Prepr. Ext. Abstr. ACS Natl. Meet. 38 (1) (1998) 91. E. Almeida, Bol. Electr. Corros. 81 (1975) 34. A. Toussaint, M. Piens, E. Almeida, Double liaison-Chimie des Peintures, DOLIA XXV, vol. 271, 1978, p. 87. A.M. Berensen, PCE (1998) 24. D. Crisp, The role of the biologist in antifouling research, in: R.F. Acker, Brown, J.R. De Palma, W.P. Iveson (Eds.), Proceedings of the Third I.C.M.C.F., Northwestern University Press, Evanston, IL, 1973, p. 88. M.G. Callow, J.A. Callow, Biologist 49 (2002) 1. G.H. Young, G.W. Gerard, W.K. Schneider, Ind. Eng. Chem. (1943) 432. C. Dallari, La Rivista del Colore, Verniciatura Industrial 10 (1112) (1977) 241. C. Dallari, La Rivista del Colore, Verniciatura Industrial 10 (1516) (1977) 417. P. Wolf, M. Schriel, The possibilities of exposure of antifouling paints in Curacau, Dutch Lesser Antilles, TNO, Report 56C (C11,19), November 1963, p. 7. S. Kiil, et al., J. Coat. Technol. 74 (929) (2002) 45 (Err. in J. Coat. Technol. 74 (932) (2002) 89). J.D. Ferry, D.E. Carritt, Ind. Eng. Chem. Res. 38 (6) (1946) 612. V.J.D. Rascio, B. Giudice, B. del Amo, Corros. Ver. 8 (1) (1988) 87. S. Kiil, et al., Ind. Eng. Chem. Res. 40 (18) (2001) 3906. S. Kiil, et al., Ind. Eng. Chem. Res. 80 (2002) 45. C. Hong-Xi, et al., Fujian Shifan Daxue Xuebao 4 (2) (1988) 61. L.R.A. Capurro, in: D.E. Grifth (Ed.), Oceanography for Practising Engineers, Barnes Noble Inc., New York, 1970, p. 67. C.L. Pickard, W.J. Emery, Descriptive Physical Oceanography: Na Introduction, Pergamon Press, Oxford, UK, 1982, p. 68. D.M. Yebra, S. Kiil, K.D. Johansen, POC 50 (2004) 75.

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