Industrial Crops and Products 65 (2015) 506–514

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Industrial Crops and Products

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Modified tannin extracted from black wattle tree as anenvironmentally friendly antifouling pigment

Rafael S. Peresa,∗, Elaine Armelinb,c, Carlos Alemánb,c, Carlos A. Ferreiraa

a LAPOL/PPGE3M, Universidade Federal do Rio Grande do Sul, Av. Bento Goncalves 9500, 91501-970 Porto Alegre, Brazilb Departament d’Enginyeria Química, ETSEIB, Universitat Politècnica de Catalunya, Av. Diagonal 647, 08028 Barcelona, Spainc Centre for Research in Nano-Engineering, Universitat Politècnica de Catalunya, Campus Sud, Edifici C’, C/Pasqual i Vila s/n, Barcelona E-08028, Spain

a r t i c l e i n f o

Article history:Received 16 July 2014Received in revised form 6 October 2014Accepted 19 October 2014Available online 8 November 2014

Keywords:AntifoulingCoatingsTanninBlack wattleEnvironmentally friendly

a b s t r a c t

The use of modified black wattle tannin as an antifouling pigment is reported in this work. A mixtureof tannin adsorbed in activated carbon (soluble fraction of tannin) and low soluble fraction of tanninwas used as an antifouling pigment. The soluble rosin resin was used as a paint matrix. 13C NMR analysisconfirm the modification of black wattle tannin through the cleavage of tannin interflavonoid bonds. FTIRspectra indicate the presence of tannin in the formulated antifouling coating even after 7 months of itsexposure in a marine environment. Water contact angle analysis shows the hydrophilic characteristic ofthe tannin antifouling coating surface. Immersion tests at Badalona Port in the Mediterranean Sea showsthe high antifouling efficiency of the TAN coating, comparable to commercial paint, until 7 months. Theuse of a natural black wattle tannin, without its complexation with metals, can eliminate the release ofmetals and other toxic biocides to the marine environment.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Hulls and submersed vessels and structures are subject to theattachment of organisms on their surface (Redfield and Hutchins,1952). The accumulation of these organisms forms a layer calledmarine biofouling, its thickness depending on the organism species,water environment and season (Callow and Callow, 2002; Wahl,1989). Barnacles, mussels, hydroids, molluscs, tubeworms andalgae are the most common types of fouling organisms found insubmersed structures and hulls (Callow and Callow, 2002). Thepresence of fouling attached to ship hulls increases the frictionwith water, raising fuel consumption (Callow and Callow, 2002;Senda, 2009). Maintenance costs of hulls, turbines, heat exchang-ers and ducts of hydroelectric plants are also affected by fouling(Lewandowski and Beyenal, 2009). Economically, fouling causesserious financial loss to industry and, therefore, control of thisorganism accumulation is mandatory (Callow and Callow, 2002).

The most common method for protecting immersed structuresagainst fouling is the use of antifouling paints (Pérez et al., 2006).In the past, several toxicants such as arsenic and mercury oxidewere added to resin matrixes, but in the 1950s concern about healthproblems led to the elimination of these products from the market

∗ Corresponding author. Tel.: +55 51 3308 9412.E-mail address: rafael.s.peres@gmail.com (R.S. Peres).

(Readman, 2006). The development of the tributyltin (TBT) com-pound introduced one of the most effective products ever madein the antifouling market (Goldberg, 1986). Its high durability andefficiency resulted in considerable savings in the maintenance costsof ship hulls (Goldberg, 1986). However, TBT was considered oneof the most toxic biocides purposely released in the marine envi-ronment (Goldberg, 1986). Studies have shown that TBT impairsthe embryogenesis and larval development of oysters, even at verylow concentrations (i.e. 0.05 �g L−1) (Alzieu, 2000; His and Robert,1983). As a consequence, the use of organotin compounds (includ-ing TBT) on ships was forbidden by the International MaritimeOrganisation (IMO, 2001).

New alternatives to TBT were development and, currently, themost common antifouling pigment is copper oxide (Callow andCallow, 2002). Copper has been used since the 1800s, even thoughits utilisation depends of local legislation (Callow and Callow,2002). However, copper is categorised as toxic for the marine envi-ronment when its concentration exceeds some threshold limits(Flemming and Trevors, 1989). Others biocides, such as Diuron®

and Irgarol® 1051, are used together (co-biocides) with copperpigment, even though they are harmful to phytoplankton organ-isms (Devilla et al., 2005). The use of Diuron® is not allowed inthe United Kingdom while Irgarol® 1051 is limited to small water-crafts (Chesworth et al., 2004). Metallic pyrithiones are also usedas co-biocides. However, these compounds show high toxicity forsome fish species (skeletal deformities were reported) (Arai et al.,

http://dx.doi.org/10.1016/j.indcrop.2014.10.0330926-6690/© 2014 Elsevier B.V. All rights reserved.

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2009). Inside this scenario, research on environmentally friendlyantifouling agents and coatings has received increasing attention.

Many natural compounds with antifouling properties werefound in natural products, as reported by several authors. Etohet al. (2002) isolated shogaols from roots of ginger that are threetimes more active than copper sulphate. The behaviour of trans-8-shogaol is similar to that of tributyltin fluoride (TBTF) againstthe adhesion of blue mussels (Etoh et al., 2002). Etoh et al.(2003) also isolated neocurdione, isoprocurcumenol and 9-oxo-neoprocurcumenol from Curcuma aromatic and Curcuma zedoaria,using them as antifouling agents against blue mussel. Watts (1995)reported the application of natural or synthetic capsaicin mixedwith epoxy resin as an antifouling agent. In fact, the large-scaleextraction of natural antifouling agents has become one of the mainchallenges in the development of ecologic antifouling paint formu-lations (Qian et al., 2010).

Tannins are natural polyphenolic compounds associated withthe defence mechanism of plants (Hagerman and Robbins, 1987).Large amounts of these compounds are typically found in the bark,roots, wood and seeds of many trees (Hillis, 1997; Rahim andKassim, 2008). Tannins can be classified as condensed, hydrolysableand procyanidins (Amarowicz, 2007; Bate-Smith and Swain, 1962).Condensed tannins are formed by flavonoid units that releaseanthocyanidins in a controlled medium (i.e. alcohol solutions athigh temperatures in the presence of strong acids) (Amarowicz,2007; Hagerman Ann et al., 1997). Hydrolysable tannins are con-stituted by gallic acid and its derivatives, which can be esterified topolyols (Hernes et al., 2001). Procyanidins, which are found onlyin brown algae, are based on phloroglucinol (Amarowicz, 2007;Hernes et al., 2001). Tannins are applied in several fields, for exam-ple in surface preparation (Peres et al., 2014a, 2014b), corrosion(Matamala et al., 2000; Rahim et al., 2007, 2008), the leather tanninindustry (Onem et al., 2014), adhesives (Pizzi, 1982) and polymers(Szczurek et al., 2014).

The antifouling properties of Sargassum procyanidin werereported by Sieburth and Conover (1965). Chet et al. (1975) evi-denced that tannic acid acts as repellent of bacteria in immersedsurfaces. The presence of bacteria in immersed surfaces is fre-quently associated with the attachment of fouling species (Chetet al., 1975; Wahl, 1989). Lau and Qian (2000) studied the inhibitoryeffect of procyanidins, phloroglucinol and tannic acid on the settle-ment of Barnacles (Balanus amphitrite amphitrite). The toxicity andinhibitory effect on the fouling settlement were tested for threelarval stages, results depending on the fouling specie and chemicalcharacteristics of the phenolic compounds (Lau and Qian, 2000).Chung et al. (1998) reported an inhibition in the growth of somemicroorganisms due to the iron-chelating effect of tannic acid. Thetannic acid removes iron ions in solution, which are essential forthe growth of some aerobic microorganisms (Chung et al., 1998).Slabbert (1992) also described the chelating effects of condensedtannins with several metals, including iron, copper, aluminium,cobalt, vanadium, zinc and nickel. According to Slabbert (1992),the condensed tannins are able to form chelates with metals due tothe presence of OH groups in ortho position on the flavonoid B-ring. Some works reported the utilisation of tannates as antifoulingagents in coating formulations (Bellotti et al., 2012a, 2012b; Pérezet al., 2006, 2007; Stupak et al., 2003). Tannates are organometal-lic compounds derived from the reaction of tannins and metallicsalts (Slabbert, 1992). Pérez et al. (2006, 2007) used the copperand aluminium tannates (from quebracho tannin) in the formula-tion of antifouling coatings. Bellotti et al. (2012a, 2012b) reportedthe utilisation of zinc tannates obtained from the reaction of taraand quebracho tannins with zinc nitrate. Stupak et al. (2003) foundthat aluminium tannate (from quebracho, chestnut and mimosatannins) has a narcotic effect on nauplii of Balanus amphitrite. Dueto the high solubility of tannin in water, the syntheses of tannates

with zinc, copper and aluminium are necessary to decrease tanninsolubility (Bellotti et al., 2012a).

The aim of this study is to investigate the use of a modifiedblack wattle tannin (extract from Acacia mearnsii) as an environ-mentally friendly antifouling agent. The tannin was been modifiedin an alcoholic medium at high temperatures with the presenceof hydrochloric acid, following by adsorption of its soluble frac-tion in activated carbon. The adsorbed and low soluble fractions ofblack wattle tannin were used as pigment. The antifouling coatingswere formulated with a natural soluble matrix (rosin). This methodavoids the utilisation of metals, enabling the utilisation of pure tan-nin, which contributes to the formulation of an environmentallyfriendly coating.

2. Experimental procedure

2.1. Materials

Black wattle tannin (TANAC, Brazil), HCl (Synth, Brazil), abso-lute ethanol (Synth, Brazil) and activated carbon powder (Delaware,Brazil) were used in the preparation of antifouling pigment. Coat-ings were prepared using rosin grade WW (RB Sul, Brazil) asmatrix, oleic acid (Sigma–Aldrich, USA) as plasticiser and methylethyl ketone (MBN chemicals, Brazil) as solvent. The commercialantifouling coating Micron® Premium (Akzo Nobel, USA) was usedas a control of the antifouling activity and the two-componentepoxy primer Intergard 269 (Akzo Nobel, USA) was used as ananticorrosive primer and blank.

2.2. Black wattle tannin modification and antifouling pigmentpreparation

The modification of black wattle tannin was based on Swainand Hills work (Swain and Hillis, 1959), even though it was withsome variations. Firstly, 10 g of black wattle tannin was dissolvedin 180 mL of absolute ethanol followed by addition of the 10 mLof deionised water. The mixture was stirred magnetically untilhomogenisation and 10 mL of concentrated hydrochloric acid wasadded carefully (drop-by-drop) to the mixture. After the completeaddition of the hydrochloric acid, the mixture was heated at 70 ◦Cuntil the almost complete evaporation of the alcoholic solution.Then, 200 mL of deionised water was added followed by vigorousstirring. After dissolution of the soluble fraction of black wattle tan-nin, 20 g of activated carbon powder was added and stirred for 1 h.At the end of the adsorption process, the pigment formed by thesoluble tannin fraction (adsorbed in activated carbon) and low sol-uble tannin fraction was filtered in a Büchner funnel and dried for24 h at 60 ◦C.

Chemical structure modification of the tannin after hydrolysiswas verified by 13C NMR and FTIR spectroscopies. 13C NMR spec-tra were performed using a 300 MHz Bruker AMX300 spectrometeroperating at 75.5 MHz. The soluble fraction of black wattle tanninwas diluted in DMSO-d6 and the internal standard was tetram-ethylsilane. FTIR spectra were recorded using an FTIR 4100 Jascospectrophotometer coupled with an attenuated total reflectionaccessory (Specac model MKII Golden Gate Heated Single ReflectionDiamond ATR). Spectra were obtained after 32 scans at a resolutionof 4 cm−1, in a spectral range of 600–4000 cm−1, in transmittancemode.

2.3. Antifouling coating preparation

Initially, 48 g of rosin flakes and 3 g of oleic acid were dis-solved in 50 mL of MEK. Then, the dissolved rosin was added inthe jacketed reactor of a Dispermat N1 (VMA-Getzmann GmbH ofReichshof, Germany) disperser equipped with a Cowles disc. The

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rosin was dispersed for 10 min before the addition of the antifoul-ing pigment (tannin adsorbed in activated carbon and low solubletannin fraction). Next, 27 g of antifouling pigment and 50 mL ofMEK were added slowly. The mixture (coating) was dispersed at4000 rpm for 2 h. After the dispersion process, the pigments of theantifouling coating were milled on a Dispermat SL-12 ball mill(VMA-Getzmann Gmbh of Reichshof, Germany). More MEK wasadded in the mill process according to the system demand (viscos-ity adjust). The mill process stopped when the pigments reached asize between 25 and 15 �m (6 and 7 Hegman).

2.4. Sample preparation and characterisation

Commercial, antifouling and blank coatings were applied overAISI 1010 steel panels with a size of 25 cm × 20 cm × 1 cm. Beforethe coatings’ application, the surfaces of steel panels were treatedas follows: degreasing with acetone; polishing with sandpaper(grain size #150); cleaning with water; and degreasing with ace-tone again. After their preparation, the steel panels were drilled ineach corner for subsequent fixation. Next, the sides of each panelwere painted with anticorrosive primer to protect the steel fromthe marine environment. Commercial (COM) and tannin-basedantifouling (TAN) coatings were only applied one side of the panel.All coatings were applied by brush and dried at room tempera-ture for 48 h before immersion tests. The dry film thickness wasmeasured with a Byko-7500 test unit (BYK Gardner, Germany).Measurements were repeated twelve times on different areas ofthe sample. The average thickness with the corresponding stan-dard deviation obtained for the COM, blank and TAN coatings was214 ± 13 �m, 39 ± 4 �m and 254 ± 14 �m, respectively.

2.5. Measurements

Scanning electron microscopy (SEM) analyses were carried outusing a focused ion beam Zeiss Neon 40 microscope operating at5 kV. Optical microscopy images were obtained using a Dino-lite(model AD7013MT) USB digital microscope. FTIR analyses wereperformed using a Bomem Michelson MB100 FTIR spectropho-tometer with a resolution of 4 cm−1 in the absorbance mode. Thewater contact angle of the formulated TAN coating was measuredusing an optical equipment model OCA 15E (Dataphysics Instru-ments), equipped with a 500 �L precision syringe (DS 500GT) andSCA 20 software for data processing. The contact angle was mea-sured on six different areas of the sample, the average value beingused to determine the surface wettability.

2.6. Immersion tests

COM, TAN and blank samples were immersed in the Mediter-ranean Sea at Badalona Port (41◦26′08.4′′ N, 2◦14′33.0′′ E) in Spain.The immersion tests were carried out between April and Novemberof 2013, a period of intense activity of some fouling organisms(Anil et al., 1995; Chiu et al., 2005). Panels were fixed with nylonstraps on a poly(vinyl chloride) support. The poly(vinyl chloride)support was built with tubes covered by polyurethane foam toavoid contact between samples and dock walls. The support wasimmersed in water to a depth of approximately 60 cm according tothe ASTM D3623 standard (ASTM, 2004) and Pérez et al. (2006)work. The panel aspect and degree of fouling attachment wereverified every month. The fouling degree was evaluated accord-ing to fouling covering, where 100% means complete coverage ofthe panel by organisms and 0% means total absence (ASTM, 2011).According to ASTM D3623 stipulations, the fouling covering at dis-tances lower than 1.3 cm from the panel edges was not consideredfor the calculation of the fouling degree (ASTM, 2004).

3. Results and discussion

3.1. Mechanism of the antifouling coating functioning

As mentioned before, the metal chelating and antifouling activ-ity of tannins was reported in the literature by some authors(Bellotti et al., 2012a, 2012b; Chet et al., 1975; Chung et al., 1998;Lau and Qian, 2000; Pérez et al., 2006, 2007; Sieburth and Conover,1965; Slabbert, 1992; Stupak et al., 2003). The bacteria repulsion(Chet et al., 1975), narcotic (Stupak et al., 2003), settlement-inhibitory (Lau and Qian, 2000) and metal chelating (Chung et al.,1998) effects of tannins were considered in the mechanism of theantifouling coating functioning.

Fig. 1 shows a hypothetical mechanistic scheme of TAN func-tioning in the marine environment. Initially, bacteria and foulingorganisms approach the substrate to find a tannin-rich surface. Theantifouling coating contains both tannin adsorbed into the acti-vated carbon pores and low soluble tannin fractions, which repulsesthe bacteria, immobilising the fouling larvae and inhibiting thefouling settlement (Chet et al., 1975; Lau and Qian, 2000; Stupaket al., 2003). The complexation of metals was also considered as thedecrease in concentration of some essential metals for microorgan-isms can inhibit their adhesion on the surface (Chung et al., 1998).Simultaneously, the rosin matrix is launched into the water envi-ronment and a new antifouling layer rich in tannin appears. Theformation of soluble resinates with Na+ and K+ ions in water isresponsible for the solubilisation of the rosin (Yebra et al., 2005).

3.2. Black wattle tannin modification

The complexity of tannins’ chemical forms requires techniquesas NMR to identify the many structures of these compounds (Ucaret al., 2013). The 13C NMR technique was used to identify the chem-ical structures of black wattle tannin before and after hydrolysis.

The 13C NMR spectrum of black wattle tannin in DMSO-d6 isshown in Fig. 2. The inset in Fig. 2 represents a schematic struc-ture of condensed tannin, where R1 and R2 can be OH or H. IfR1 = OH and R2 = H the chemical structure corresponds to pro-cyanidin (Chai et al., 2012) while if R1 = R2 = OH, the chemicalstructure is prodelphindin (Chai et al., 2012).

Chemical shifts reported for different polyphenols (Castillo-Munoz et al., 2008; Chai et al., 2012; Cren-Olivé et al., 2002;Czochanska et al., 1980; Davis et al., 1996; Navarrete et al., 2010;Pizzi and Stephanou, 1993; Pizzi, 1994; Porter et al., 1985; Ucaret al., 2013; Watanabe, 1998; Wawer et al., 2006; Zhang et al.,2010; Zhao et al., 2012) were used to identify the peaks obtained forblack wattle tannin (Fig. 2). The peak at 145 ppm corresponds to theC3′ and C4′ of B-ring, while peaks at 156–154 ppm are associatedto the C5 and C7 (A-ring) bonded to–OH (Navarrete et al., 2010;Zhao et al., 2012). The chemical shifts at 133 and 118 ppm belongto the C1′ and C5′ atoms of B-ring, respectively (Navarrete et al.,2010; Zhao et al., 2012). The peak at 115 ppm corresponds to theC4 C8 interflavonoid bond while those at 107–105 correspond tothe C4 C6 interflavonoid bond (Navarrete et al., 2010; Zhao et al.,2012). The peak at 97 ppm corresponds to the terminal C6, C8 andC10 of A-ring. The chemical shifts between 68 and 75 ppm corre-spond to the C3 atom and C4 C8 and C4 C6 interflavonoid bonds.It should be mentioned that characteristic peaks of glycosides alsoappear in the latter region of the spectrum and, therefore, can over-lap the flavonoids peaks (Castillo-Munoz et al., 2008; Chai et al.,2012; Ucar et al., 2013; Watanabe, 1998; Zhang et al., 2010). Thebands at 75–77 ppm correspond to C2 (C-ring) in cis position whilethose at 82 ppm involve the trans form (Czochanska et al., 1980;Ucar et al., 2013; Zhang et al., 2010). The band at 64.17 ppm corre-sponds to C3 while terminal C4 appears in 29–27 ppm (Czochanskaet al., 1980; Zhang et al., 2010).

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Fig. 1. Hypothetical scheme of the TAN coating action in the marine environment. The repulsion of bacteria, immobilisation of fouling larvae, inhibition of fouling settlementand metal chelating were considered as the antifouling effect of black wattle tannin (Chet et al., 1975; Chung et al., 1998; Lau and Qian, 2000; Stupak et al., 2003).

Despite of polymerised black wattle tannin being highly solublein water, first attempts for its adsorption in the activated carbonwere unsuccessful. Accordingly, hydrolysis of tannin was proposedto facilitate the adsorption step. The cleavage of interflavonoid bondby acid-catalysed reaction decreases the molecular weight of tan-nin leading to the formation of highly soluble (anthocyanidins) andpoorly soluble products (Hillis, 1997; Porter et al., 1985; Swain and

Hillis, 1959). The soluble products were successfully adsorbed inactivated carbon and tested as antifouling pigment together withthe poorly soluble products. The 13C NMR spectrum of black wat-tle tannin after hydrolysis (soluble fraction diluted in DMSO-d6)is given in Fig. 3. The inset in Fig. 3 represents a schematic struc-ture of black wattle tannin after hydrolysis, where R1, R2 and R3can be OH or H. As examples, if R1 = OH, R2 = OH and R3 = H

Fig. 2. 13C NMR spectrum of black wattle tannin in DMSO-d6. The inset represents a schematic structure of condensed tannin. Procyanidin corresponds to R1 = OH andR2 = H, whereas prodelphindin refers to R1 = R2 = OH (Chai et al., 2012).

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Fig. 3. 13C NMR spectrum of black wattle tannin in DMSO-d6. The inset represents a schematic structure of condensed tannin. If R1 (in dashed bond) = R2 = OH and R3 = Hthe chemical structure corresponds to catechin, whereas if R1 (in wedge bond) = OH = R2 = OH and R3 = H the chemical structure refers to epicatechin (Chai et al., 2012).

the chemical structure corresponds to catechin, whereas if R4 (inwedge bond) = R2 = OH and R3 = H, the chemical structure refersto epicatechin.

The characteristic chemical shifts of black wattle tannin afterhydrolysis (Fig. 3) were identified according to the literature(Castillo-Munoz et al., 2008; Chai et al., 2012; Cren-Olivé et al.,2002; Czochanska et al., 1980; Davis et al., 1996; Navarrete et al.,2010; Pizzi and Stephanou, 1993; Pizzi, 1994; Schmidt et al., 2004;Ucar et al., 2013; Watanabe, 1998; Wawer et al., 2006; Zhang et al.,2010; Zhao et al., 2012). Although many peaks are similar to theblack wattle tannin spectrum, some changes are detected at sig-nals that correspond to interflavonoid bonds. The chemical shiftat 115 ppm (C4 C8 interflavonoid bond) and at 107–105 (C4 C6interflavonoid bond) disappear in the spectrum of tannin afterhydrolysis. Some bands at 68–75 ppm related to the C4 C8 and

Fig. 4. FTIR spectra of (a) rosin, (b) the TAN coating before immersion and (c) theTAN coating after 7 months of immersion at Badalona Port (the Mediterranean Sea).The inset shows the FTIR spectrum of black wattle tannin.

C4 C6 interflavonoid bonds disappear as well. Another impor-tant modification was the appearance of a new chemical shift at19 ppm, which correspond to the free C4 in the A-ring (Zhao et al.,2012). A displacement in the 97 ppm peak, which correspondsto terminal C6 and C8, is also observed. These modifications inthe spectrum corroborate the cleavage of interflavonoid bonds,as expected from the hydrolysis (Hemingway and McGraw, 1983;Kennedy and Jones, 2001; Swain and Hillis, 1959).

3.3. Characterisation of antifouling coating surface by FTIR

The chemical modifications undergone on the TAN coatingsurface after 7 months of immersion at Badalona Port (the

Fig. 5. Water droplet representation of the TAN coating.

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Fig. 6. Photographs of (a) blank, (e) TAN and (i) COM samples before immersion at Badalona Port (the Mediterranean Sea); (b) blank, (f) TAN and (j) COM after 2 months; (c)blank, (g) TAN and (k) COM samples after 4 months; and (d) blank, (h) TAN and (l) COM samples after 7 months. The dimension of each steel panel is 25 cm × 20 cm × 1 mm.

Mediterranean Sea) were verified by FTIR. The FTIR spectra of tan-nin, rosin, TAN and TAN after 7 months of immersion are displayedin Fig. 4. The spectrum of pure rosin (Fig. 4a) shows the band of the

CO from diterpenic acids at 1690 cm−1, while peaks at 1386 cm−1

and 1445 cm−1 correspond to–CH3 bending vibration (Azémardet al., 2014). Peaks at 1238 cm−1 and 963 cm−1 refer to COOH.The peaks at 1613, 887, 830 and 706 cm−1 are assigned to aromaticgroups while the peak at 1183 cm−1 is attributed to saturated C Cor CH in an aromatic ring (Font et al., 2007).

As expected, the peaks at the FTIR spectrum of TAN beforeimmersion (Fig. 4b) are similar to rosin and oleic acid due to entrap-ment of pigments inside the matrix. However, after 7 months ofimmersion (Fig. 4c) the peaks of the TAN coating show impor-tant differences. The bands at 1547, 1390 and 1145 cm−1, whichcorrespond to C C of aromatic groups (Hussein et al., 2009), COHdeformation of phenols (Jensen et al., 2008) and aromatic �C O(Soto et al., 2005), respectively, appear in TAN (after immersion)and black wattle tannin (inset of Fig. 4) spectra. These bands arerelated to polyphenolic compounds, confirming that part of therosin matrix was launched into water and the tannin was exposed.

Another important result of the FTIR analysis corresponds to thepresence of tannin in the TAN coating, even after 7 months ofimmersion.

3.4. Water contact angle of the antifouling coating surface

In order to get more information about the behaviour of the TANcoating surface in the marine environment, water contact measure-ments were carried out. The mean contact angle measured for TANwas 72.9 ± 0.7◦ (Fig. 5), indicating good surface wettability (Yuanand Lee, 2013). This low angle value favours the action of the TANcoating since water is enabled to penetrate inside the pores of theactivated carbon. Hydrophilicity is also important for the solubili-sation of the low soluble tannin fraction.

3.5. Immersion tests

The painted panels were immersed in Badalona Port (theMediterranean Sea) to verify the antifouling efficiency of the TAN

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Fig. 7. Optical Micrographs of blank (a, c and e), TAN (d and f) and COM (b) samples after immersion tests.

coating. The TAN, COM and blank samples were immersed togetherat the same place and time.

Fig. 6 shows the aspect of the tested panels before and after2, 4 and 7 months of immersion. The blank panel (Fig. 6b) showsmany points of fouling attachment after only two months of immer-sion, while both the COM (Fig. 6j) and TAN (Fig. 6f) coatings remainwithout fouling after this period of time. In the fourth month, thearea covered by fouling on the blank panel (Fig. 6c) increased sig-nificantly, while no hard or soft fouling was attached to the TAN(Fig. 6g) or COM (Fig. 6k) coatings. Callow and Callow (2002) classi-fied the fouling degree as hard and soft depending on the attachedfouling organisms.

The COM coating (Fig. 6l) remained without attached foulingduring the whole immersion assay (7 months), even though manyfailures, such as bubbles and detached areas, were found in thecoating. Conversely, the blank panel (Fig. 6d) was completely cov-ered by fouling after 7 months of immersion. In the TAN coating(Fig. 6h), only soft fouling (some algae and hydroids) was detectedon a few points of the surface. Indeed, detailed inspection revealsthat this phenomenon is exclusively localised at some failure sur-face regions (detached areas). These observations clearly reflect theexcellent antifouling properties of the TAN coating. The formationof a stable metallic complex between iron and black wattle tannin

also improves the durability of the formulated coating (Slabbert,1992).

3.6. Microscopy analysis

In order to analyse the surface of the TAN coating after sevenmonths of immersion, analyses based on SEM and optical micro-scopies were carried out. Fig. 7 shows optical micrographs of thesurface of the blank panel. As can be seen, hard fouling is presenton the entire surface (Fig. 7a). Thus, tubeworms, barnacle, spongesand mussels are detected (Fig. 7c). The thickness of the fouling layeron the blank panel reached 7 mm in some areas, as is evidenced inFig. 7e.

Fig. 7b displays the optical micrograph of the COM surface, whileFig. 7d and f shows optical micrographs of the TAN surface. Thecracks and detached parts of the TAN and COM surfaces appear afterdrying. The latter refers to the elimination of moisture in dry envi-ronmental conditions from wet samples after 7 months of exposurein the marine environment and not the drying step after painting.However, the COM sample already showed some detached partsbefore this drying step (Fig. 7b). Fig. 7b reflects the absence offouling on the COM surface. Fig. 7d reveals the presence of lightfouling (algae) at some points on the TAN surface, as observed in the

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Fig. 8. SEM micrographs of the TAN antifouling coating: (a) area covered by soft fouling (algae); (b) diatoms on the TAN surface; (c) and (d) porosity of the TAN coatings atdifferent magnifications.

photograph displayed in Fig. 6h. However, hard fouling is absent,as is clearly evidenced by comparing the two sides of the panel(Fig. 7f). Thus, the thickness of the side painted with the TAN coat-ing (right side in Fig. 7f) is considerably smaller than that of theback side (left in Fig. 7f), which was not painted with antifoulingcoating and presents significant amounts of hard fouling.

SEM micrographs of the TAN coating surface after 7 monthsof immersion are displayed in Fig. 8. As observed in the opticalimage (Fig. 7d), there is only soft fouling on some points on the TANsurface. Fig. 8a shows the TAN sample with some algae brancheson the surface. At high magnification, the presence of unicellu-lar algae called diatoms is detected (Fig. 8b). According to Callowand Callow (2002), diatoms are species of Amphora able to growon some copper antifouling coating surfaces. Diatoms can form aslime (biofilm) whose average thickness can reach values as highas 500 �m (Callow and Callow, 2002). The antifouling efficiency ofthe TAN coating is reaffirmed by SEM analysis, because only softfouling is detected, as occurs on the copper antifouling coatings.In the areas with no algae, the high porosity of the TAN coating isverified (Fig. 8c and d). This porosity is important for the properreleasing of black wattle tannin.

4. Conclusions

The antifouling activity of a tannin-based antifouling coating(TAN) was proved in this work. The modification of black wattle tan-nin through the cleavage of tannin interflavonoid bonds has beenconfirmed by 13C NMR spectroscopy. Furthermore, the presence oftannin in the formulated antifouling coating, even after 7 monthsof its exposure in the marine environment, has been evidencedby FTIR spectroscopy. Water contact angle analysis has shown thehydrophilic nature of the tannin antifouling coating surface.

Immersion tests at Badalona Port in the Mediterranean Sea havereflected the high antifouling efficiency of the TAN coating up to 7months, which is comparable to that of the commercial paint. Pho-tographs and optical images of blank samples have evidenced thehigh activity of fouling organisms in the tested area. Microscopyanalyses together with photographs of immersed samples haveconfirmed the presence of only soft fouling on the TAN surface. Asa final remark, results obtained in this work indicate that the use

of natural black wattle tannin, without complexation with metals,can eliminate the release of metals and other toxic species into themarine environment. Thus, the negative impact on the environ-ment can be drastically reduced or even eliminated with the use oftannins as antifouling agents.

Acknowledgments

The authors thank the Brazilian government agencies CNPq andCAPES (process BEX 13736124) which provided the financial sup-port for this study and the scholarship for R.S Peres. Financialsupport for E.A. and C.A. comes from MICINN and FEDER (MAT2012-34498), and the Generalitat de Catalunya (research group 2009 SGR925) is gratefully acknowledged. Support for the research of C.A.was received through the “ICREA Academia” prize for excellence inresearch funded by the Generalitat de Catalunya.

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