J. Coat. Technol. Res., 10 (1) 29–36, 2013 DOI 10.1007/s11998-012-9456-0

REVIEW PAPER

A brief review of environmentally benign antifouling and foul-release coatings for marine applications Pascal Buskens, Marie¨lle Wouters, Corne´ Rentrop, Zeger Vroon

Ó American Coatings Association & Oil and Colour Chemists’ Association 2012 Abstract Antifouling coatings for ship hulls are a very important topic in coating research. They are essential with respect to fuel consumption of ships: without antifouling coating, biological species start to adhere to the ship’s exterior, leading to a gradual increase in fuel consumption. To date, the working principle of most of the paint systems applied is based on slow release of toxins in time (self-polishing coatings). In this article, we discuss the environmental impact of marine antifouling coatings based on quantitative data available from literature. In addition, we critically review hydrophilic antifouling and hydrophobic foul-release coatings as toxin-free alternatives and discuss their potential for replacing self-polishing coatings. Keywords Antifouling coating, Foul-release coating, Switchable coating, Environmentally benign

Introduction Marine bio-fouling is a highly complex process and can involve a wide variety of up to 4000 different species and organisms.1,2 Typically, the process is divided into two main stages: micro- and macrofouling. During microfouling, a bio-film is formed and bacteria start to adhere. In the macrofouling stage, larger organisms start to attach. Marine bio-fouling is still a subject of research. Progress in this research area has been summarized and discussed in a variety of review articles on marine antifouling coatings in the past few years3–9 and will consequently not be discussed in more detail here. Antifouling coatings for ship hulls, P. Buskens (&), M. Wouters, C. Rentrop, Z. Vroon TNO, De Rondom 1, 5612 AP Eindhoven, The Netherlands e-mail: [email protected]

i.e., coatings that prevent or slow down the growth of organisms on the ship’s exterior, are a very important topic in coating research. They are essential with respect to fuel consumption of ships: without antifouling coating, biological species start to adhere on the ship’s exterior leading to a gradual increase in fuel consumption.10–12 Figure 1 displays a ship hull without antifouling coating comprising barnacles and other marine organisms on the exterior of the ship. In a study performed by Tobias Boren for International Paint, a 300,000 dead weight tonnage very large crude carrier (VLCC) consuming 100 tons of heavy fuel per day working at 80% activity and operating on a 5-year docking cycle was chosen as model system.14,15 An unprotected VLCC with 40,000 m2 of underwater area can gather up to 150 kg of fouling per m2 in less than 6 months at sea, leading to an enormous increase in weight and fuel consumption in time.16 Boren’s study demonstrates the environmental performance and relative cost of a benchmark antifouling coated system to an uncoated vessel with the following key output: overall fuel savings of 39,420 metric tons over a period of 15 years and consequently cost savings of about 28.5 million US dollars (at a price of 723 $ per metric ton heavy fuel oil). Furthermore, the overall amount of emitted CO2 is reduced by more than 125,000 metric tons over 15 years. Other studies show similar results.16–20 To date, the working principle of most of the paint systems applied commercially is based on slow release of toxins in time. These coatings are called selfpolishing—they slowly erode in time releasing enclosed toxic compounds such as organotins or cuprous oxide which is at low concentrations toxic to most forms of fouling.21–24 Although the antifouling performance of such systems is excellent, the amount of toxins released per ship is enormous. Assuming a coating lifetime of 5 years, a thickness of 400 lm and a toxin loading of 3 vol%, a large tanker from the VLCC

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Fig. 1: Bio-fouling on ship hull (from: European Coatings)13

class with a water contact area of about 40,000 m2 releases about 480 L of toxin over a period of 5 years. Per year, about 96 L of toxin is leached by such tankers. At any given moment, about 500 of such tankers are on seas worldwide, resulting in a yearly amount of 48,000 L of toxin leaching into the seas. Taking into account all kinds of other vessels, it is reasonable to assume that several hundred thousand liters of toxins are released into sea and river water per year. The impact of such toxins on nature can be detrimental. It was shown that tributyltin (TBT)—a very powerful toxin commonly used in self-polishing antifouling coatings—induced sex changes in dog whelks and sea snails.25–27 In 2005, studies performed at Yale University even linked TBT to whale beachings.28 Because of their detrimental impact on nature, the use of organotins such as TBT on ship hulls was completely banned in 2008.29–32 In addition, the use of other toxins in antifouling coatings is increasingly restricted by law. Although the use of copper-based paints is not yet prohibited, copper is increasingly being banned. Recently, the use of copper-based coatings on recreational boats has been banned in the ports of San Diego and Washington. This drives both science and industry to evaluate other types of antifouling mechanisms. In the past decade, several new types of antifouling coatings have been developed. In addition, foul-release coatings have been extensively studied for marine applications. In this article, the authors focus on two toxin-free systems that are commonly suggested or already applied as alternatives to self-polishing coatings: (a) hydrophilic antifouling coatings that prevent or slow down adherence of marine biological species to ship hulls and (b) low energy, hydrophobic foul-release coatings that facilitate an easy release of marine biological species. Other alternatives to self-polishing coatings like enzyme-based systems33 or coatings with covalently attached toxins34 are not discussed in this article. Please note that some of the coating systems presented below are designed for applications other

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Fig. 2: Antifouling coatings based on hydrophilic polymer brushes [red ball = biological species, black square = linker, black wave = (compressed) polymer brush] (Color figure online)

than marine antifouling, e.g., water purification membranes or medical disposables. The working principle of all coatings presented in this article, however, is of direct relevance to marine antifouling applications. In addition, the authors would like to point out that the performance of the coating systems as described below is evaluated using a variety of different testing methods. More extensive information on suitable test methodologies to determine the performance of (marine) antifouling coatings can be found in several recent review articles.35–41

Hydrophilic antifouling coatings It has been known for a long time that hydrophilic polymer brushes can prevent the adhesion of proteins to surfaces and thereby avoid the formation of a biofilm. This is attributed to the entropic repulsion of proteins by the polymer brushes in their hydrated forms (see schematic representation in Fig. 2). There are many reports, publications, and patents that deal with this subject.42–46 Potential application areas mentioned range from marine applications to medical devices and disposables. Mostly, polyethylene oxide (PEO) brush coatings are applied.47–54 In many cases, a monolayer of PEO brushes is applied to a surface via grafting.10 To graft the PEO chain to a surface, a linker has to be used (see Fig. 2). The choice of linker depends on the substrate: typically siloxane linkers are used, especially for connecting PEO to glass, ceramic, and metal surfaces. In specific cases, the use of other linking groups such as thiols can be beneficial. Although these coatings lead to a strong reduction in bacteria and protein adsorption, they are of limited use because of their fragility. They are easy to damage and directly lose their function upon minor mechanical impacts. A better applicable PEO-based system has been described by Busscher and co-workers and is reported in patents by Currie et al.55,56 They report the use of silica particles as carrier for both the PEO chains and

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polar conformation into a protein attractive, apolar conformation. These drawbacks make the application of such coatings on ship hulls very challenging.

Hydrophobic foul-release coatings The surface energy is defined as the excess energy of the molecules on the surface compared to the molecules in the thermodynamically homogeneous interior of a material or coating.74 The surface energy can be determined by contact angle measurement using different fluids. Although in general a lower surface energy implies less capability to interact spontaneously with other compounds and materials, the Baier curve shows that there is an optimum in surface energy for achieving low relative adhesion (see Fig. 3).75 Hydrophobic coatings, e.g., silicone- or fluorinebased coatings have a low-surface energy. Hence, they release accumulated bio-fouling either through the action of hydrodynamic forces generated as a vessel moves through water or through direct cleaning by hand or using robots. Webster and co-workers developed a self-stratifying coating system comprising a polyurethane resin tethered with a siloxane surface resin.76–80 The polyurethane provides desired toughness and adhesion, the siloxane moiety provides the low-surface energy needed for good fouling release properties. The combination of a polyurethane component for strength in combination with siloxanes or fluoralkyl compounds to lower the surface energy is investigated by a broad variety of research groups.81–83 Various groups investigated the effect of surface structuring on the fouling characteristics of hydrophobic coatings. Zhang and co-workers focused on three super-hydrophobic coatings with varying chemistry and surface roughness and compared these to their nonstructured equivalents.84 Although under oceanic conditions they observed that the structured coatings

Relative adhesion

the acrylic moieties. The functionalized silica particles are formulated into an UV curable resin and the acrylic moieties are used for the formation of the coating network. A multifunctional acrylate is used as binder. The resulting coatings are described as remarkably hard and stiff in the dry state (1.41 GPa hardness, 15.18 GPa reduced elastic modulus at 100 nm coating thickness on glass). At a PEO loading of 0.6 chains per nm2 the adhesion of Staphylococcus epidermis HBH 276 was reduced by more than 90%. This demonstrates that a highly crosslinked acrylate coating with integrated inorganic particles can form the base for a robust and efficient PEO-based antifouling coating. Although most articles and patents dealing with hydrophilic antifouling coatings are based on PEO systems, researchers also investigated the use of alternative polymers. The group of Bosker investigated the use of dextran.57 They grafted dextran to a polystyrene surface using polystyrene–dextran copolymers and the Langmuir–Blodgett deposition technique. At grafting densities above 0.2 chains per nm2 a homogeneous dextran brush was observed and the adsorption of bovine serum albumin (BSA) and trypsin was drastically decreased (more than 90%). The group of Chehimi described the use of poly-(2-hydroxyethylmethacrylate) (poly-HEMA) surface coatings to prevent Salmonella bacteria from adhering to glass surfaces.58 The group of Kang at the National University of Singapore described the use of poly-HEMA brushes modified with chitosan on stainless steel surfaces.59 The latter system combined the antimicrobial effect from chitosan with a polymer brush structure to insure that dead bacteria do not stick to the substrate surface and serve as a nucleation point on which other biological species would adhere. Other brush forming polymers often used are polyvinyl alcohol60,61 and poly(N-(2-hydroxyethyl)acrylamide).62 A special class of brush polymers is zwitterionic brushes. Inspired by the antibiofouling properties of blood cell membranes, polymers incorporating zwitterionic molecules like phosphatidylcholines are promising as antifouling coatings.46,63,64 Zwitterionic brushes with antifouling properties based on carboxybetain are among others described by the group of Kitano.65–67 Callow, Jiang and others describe coating systems based on polysulfobetain polymers.68–71 They grafted poly(sulfobetainmethacrylate) (PSBMA) brushes onto glass surfaces using surface initiated atom transfer radical polymerization (ATRP). Only small amounts of the green marine alga—Ulva—settled on the PSBMA surfaces and the adhesion strength of both spores and sporelings (young plants) was low. Most systems using hydrophilic brush polymers, however, suffer from four major drawbacks55,72,73: (1) their limited mechanical robustness, (2) their sensitivity toward hydrolysis, (3) their sensitivity toward oxidation, and (4) the fact that—under specific conditions—some of them can attract proteins. In the case of PEO-based systems, the latter is explained by a change in conformation of the EO segments from a protein repellent,

0

80 Surface energy (mN.m−1)

Fig. 3: Baier curve

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fouled slightly slower than their nonstructured equivalents, they could not demonstrate a significant difference in the amount of fouling uptake between the structured and nonstructured hydrophobic coatings after a prolonged time period. Wouters and co-workers used various amounts of modified sepiolite structures to create hydrophobic sol–gel-based coatings with a specific surface roughness (see Fig. 4).85 In their studies, they demonstrated that the surface roughness can have a positive effect on the formation and release of bio-films. In addition to the surface energy and surface roughness, the elastic modulus of the coating plays a role in its fouling characteristics. It is a key factor in bio-adhesion and the ability of organisms to release from a surface. Brady and Singer showed that the relative adhesion increases linearly with the square root of the elastic modulus and the critical surface free energy.86 This was later confirmed by a variety of other research groups.87–89 Hydrophobic foul-release coatings are typically less sensitive than hydrophilic antifouling coatings. First, low-surface energy coatings for marine applications are already commercially available.90 It is claimed that all vessels operating above 10 kts (1 kt = 1.85 km per hour) benefit from such a foul-release coating. Furthermore, a fuel and emission savings of 6% in comparison to biocide-containing self-polishing coatings has been claimed. Most hydrophobic, foul-release coatings suffer from three specific drawbacks: (a) adhered species are only

efficiently removed when the coated object is in movement, (b) the coatings are typically relatively soft and easily damaged, and (c) the coatings may contain leachables that can be partially responsible for the foul-release performance. This means that hydrophobic foul-release coatings are not applicable for static applications and that their performance might significantly change over time. Nonetheless, they are currently considered the best in class of toxin-free ship hull coatings.

Switchable antifouling/foul-release coatings The group of Mannari and co-workers described a stimuli-responsive antifouling system.91 They designed their system in a way that in water the surface becomes hydrophilic and in air it switches to hydrophobic. They use polyurethane coatings with varying weight percentages of hydrophilic (PEO), hydrophobic (CF3– (CF2)6–CH2–), and amphiphilic brushes (copolymers containing both moieties). In water, the PEO brushes stick out and strongly reduce the adhesion of proteins and bacteria. Over time, however, small quantities of bio-material start to adhere and need to be removed in a cleaning step. For cleaning purposes, the coated vessel is taken out of the water resulting in a switch from a hydrophilic to a hydrophobic surface. This enables an easy and efficient cleaning of the ship’s exterior. Coatings developed by Wouters et al. dem-

Fig. 4: Antifouling coatings with high degree of surface roughness as reported by Wouters and co-workers85

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onstrate a similar switching behavior at low levels of crosslink density.92,93 Although this area of research is still embryonic, it combines both working principles of fouling prevention and foul release in a smart way. Fouling is prevented by hydrophilic brush polymers while immersed in water and the release of fouling is facilitated in dry conditions. The latter makes cleaning of the ship’s hull easier. An intrinsic difficulty of these coatings, however, is the trade-off between the ability to switch and the mechanical stability of the system. At low crosslink densities, the switching process works well but the coatings are quite fragile. At high crosslink densities, the coatings are more robust but switching becomes increasingly difficult. More research on such smart, switchable systems is expected in the near future.

Conclusions Antifouling coatings are essential for a ship’s performance: they are designed to protect the ship’s hull from fouling with bio-organisms. Unprotected vessels can gather huge amounts of bio-fouling per square meter immersed area in a relatively short period of time (up to 150 kg within 6 months). This leads to more fuel consumption and consequently more carbon dioxide emission. Traditionally, self-polishing coatings were used to prevent bio-fouling on ship hulls. Their working principle is based on a gradual coating erosion and release of toxic biocides like organotin and copper compounds. Most of these compounds have a detrimental impact on the environment; some of them have already been banned. In the past decade, two types of toxin-free, environmentally benign antifouling coatings have been developed: hydrophilic polymer brush and hydrophobic low-surface energy coatings. It has been demonstrated by various research groups that both systems lead to a significant reduction in bio-fouling and facilitate an easy release of foulants, respectively. The latter system has already found its way into commercial products. For the future, we expect more research on environmentally benign, marine antifouling coatings; in the marine environment, all surfaces are affected by the attachment of fouling organisms, so not only ship hulls but also underwater cameras, fishing nets, and so on. There is an ongoing need to constantly improve the performance of antifouling coatings and to raise environmental awareness. The ever tighter legislation regarding safety and environmental protection is driving the development of an eco-friendly marine coating solution. For the hydrophilic polymer brush coatings, finding more robust and durable systems will be the topic of focus. In the case of hydrophobic foul-release coatings, the combination of surface hydrophobicity with a surface structure seems promising. Furthermore, smart switchable coating layers using responsive polymer systems will be an area of focus.

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