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Development of hybrid antifouling paints
Progress in Organic Coatings 87 (2015) 10–19
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Development of hybrid antifouling paints ¨ Karine Réhel, Isabelle Linossier Fabrice Azemar ∗ , Fabienne Fay, Laboratoire de Biotechnologie et Chimie Marines, Univ. Bretagne-Sud, EA 3884, LBCM, IUEM, F-56100 Lorient, France
a r t i c l e
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Article history: Received 3 October 2014 Received in revised form 8 March 2015 Accepted 13 April 2015 Keywords: Antifouling Hybrid paints Poly(-caprolactone) Poly(dimethylsiloxane) Biofouling Binder
a b s t r a c t The objectives of this study are to measure the efficiency of a hybrid system for antifouling paint and to observe the influence of the physico-chemical properties of the binder on the antifouling activity. Poly(caprolactone) (PCL) and poly(dimethylsiloxane) (PDMS) homopolymers are already used as binder for different antifouling strategies. The use of the block copolymer should allow to mix the properties of erosion and hydrophobicity to obtain a more efficient paint with a reduced environmental impact. Paints containing triblock copolymer were immersed in seawater in Lorient harbour to evaluate their antifouling activities. The water absorption, the degradation and the surface properties of the copolymer were measured in distilled water to understand the influence of the binder characteristics in the resulting antifouling properties. The hybrid paints have shown efficiency close to a commercial paint during their immersion in situ in spite of inadequate static conditions of test. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Antifouling paints are the main system used against the development of organisms (bacteria, algae, mollusc, etc.) on immerged surface and the most studied. The colonization by organism and their secretion form the biofilm [1]. The biofouling development on the surfaces is an economic and environmental problem [2]. It increases the surface roughness, the material degradation and the fuel consumption of ships. Moreover, it should be responsible for the introduction of invasive species [3]. Since the 19th century and the use of first antifouling paints [4], lot of researches have been done to increase efficiency against the biofouling and the lifetime [5]. However, these improvements have been done to the detriment of the environment. The more known example is the introduction of tributyltin-based compounds (TBT) as biocide in paints formulation to obtain cheap paints with an efficiency of 5 years [6]. Unfortunately, its use has caused many troubles on marine species due to the accumulation and severe toxicity of TBT [7,8]. To substitute paints with TBT, two main techniques were used: the biocide release strategy based on the incorporation of natural biocides or synthetic biocides less harmful for the environment [9] and the fouling release strategy based on paints with physico-chemical surface properties to limit durable colonization
∗ Corresponding author. Tel.: +33 681758801. E-mail address: [email protected] (F. Azemar). http://dx.doi.org/10.1016/j.porgcoat.2015.04.007 0300-9440/© 2015 Elsevier B.V. All rights reserved.
by organisms [10]. But, the development of new biocides and new paints have greatly increased the paints price without increase their efficiency. Moreover, both kinds of paints have shown other disadvantages. For the coatings with biocides, the new European and international regulations on chemical products will limit their uses for environmental application. Concerning paints with surface properties, it is their inefficiency during static periods is problematic [11,12]. Indeed the biofilm is removed by hydrodynamic stress like navigation for ships or a mechanical cleaning. In the aim to develop a new kind of more environmental antifouling paint, two polymers seem appropriate. A polyester, the poly(-caprolactone) (PCL) which has been already used as binder for antifouling paints [13,14]. Its biodegradable properties [15,16] are well known and allow to release biocide. Furthermore, polyesters seem to be a perfect candidate for a new generation of antifouling paints, they are already used for environmental application [17] and they are cheap. Another interesting polymer already used for antifouling paints is a silicone, the poly(dimethylsiloxane) (PDMS). It is the most found silicone in commercial application and it has done good results as binder in paints with surface properties [18,19]. Moreover, PDMS homopolymers are already used by some companies (Chugoku Marine Paints, PPG, etc.) for their antifouling paints [5]. Buchholz and Mülhaupt have already synthesized a triblock copolymer containing these two polymers [20]. Moreover the block copolymer poly(-caprolactone)-blockpoly(dimethylsiloxane)-block-poly(-caprolactone) (PCL-b-PDMSb-PCL) obtained by this way have shown interesting properties of hydration and degradation in a previous work of our group [21].
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Fig. 1. Synthesis of the binder by ring opening polymerization of -caprolactone.
In the present work, the linear triblock copolymer was incorporate as binder in antifouling paint to measure the efficiency of this system in seawater and to evaluate the influence of polymer properties on the antifouling activity. Furthermore, this work have to shown the interest to develop a hybrid system who should allow to limit the quantity of biocide in the paints and to have a high efficiency in all conditions due to the properties of both polymers.
2. Experimental 2.1. Materials The monomer of -caprolactone (CL) and the catalyst stannous octanoate (Sn(Oct)2 ) were purchased from Aldrich Co. The polydimethylsiloxane Tegomer® H-Si 2311 (Mn ≈2300 g mol−1 ) was kindly supplied by Evonik Industries, Germany. All the products were used as received.
2.2. Binders synthesis Binders are triblock copolymers with poly(-caprolactone) external blocks and a polydimethylsiloxane core block. The copolymers were synthesized by ring opening polymerization of -CL in the presence of PDMS with Sn(Oct)2 as catalyst (Fig. 1). The copolymer composition was varied by changing the feed ratio of the reactants [CL]/[PDMS] (270:1; 60:1; 30:1 n/n, respectively) to study the effect of the composition on the antifouling properties. The same procedure than described in a precedent work was used [21].
3. Paint preparation To evaluate the physico-chemical and antifouling properties of the binders, paints were formulated with the three copolymers. All the components presented in Table 1 were dispersed under vigorous agitation during 30 min. Then the paints were applied on polycarbonate plates for the studies of the physico-chemical properties and on glass–polyester composite plates for the evaluation of antifouling properties.
Table 1 Components of the paints. Solvents
Xylene MIBK
Binder
PCL-b-PDMS-b-PCL
Fillers
TiO2 ZnO CaCO3
Additives
Dichlofluanide Zinc pyrithione Copper thiocyanate Polyamide waxes
4. Measurement Differential scanning calorimetry (DSC): Calorimetric data were obtained using a Mettler-Toledo DSC 822 (Mettler Toledo, Viroflay, France). The calibration was done with indium and zinc. Aluminium pans were used and the sample mass was approximately 10 mg. The samples were first melted to 100 ◦ C (1st run/20 ◦ C min−1 ) and kept at this temperature for 2 min, then cooled to −100 ◦ C and heated up again to 200 ◦ C (2nd run/20 ◦ C min−1 ). Temperature and heat of phase transitions were determined respectively from the maxima as well as crystallization areas and melting peaks. The degree of crystallinity was estimated using Eq. (1) where Hm is the measured melting enthalpy of the polymeric part of the sample and H100% the equilibrium melting enthalpy of 100% crystalline PCL taken equal to 139.3 J g−1 [22]. Xc =
Hm × M%PCL H100%
(1)
Karl–Fisher coulometer (KF): The quantity of water in the paints during the immersion were determined by a Metrohm KF 737 equipped with a Metrohm Oven KF 707 (T = 150 ◦ C) which was used under a nitrogen flow of 200 mL min−1 . The reactant was HydranalCoulomat AG. To measure only the water in the film, the samples were wiped before inserted in the coulometer. Gel permeation chromatography (GPC): GPC chromatograms were measured by a Merck pump L-7110, two columns PLgel (Mixed-E, 3 m and Mixed-D, 5 m) from polymer laboratories, and a Sedex DEDL detector. THF was used as eluent at a flow rate of 1 mL min−1 , and the injection volume was 20 L. The calibration was done with polystyrene standards with molar masses included between 1300 and 400,000 g mol−1 (Easical PS-2) from Agilent Technologies. Before the injection, the dry paints were centrifuged at 5000 rpm during 10 min at 20 ◦ C to eliminate water. Then the supernatant was filtered with PTFE filter (0.45 m). Contact angle measurement: Measurements were taken at room temperature with a contact angle metre Digidrop GBX. Ten droplets of 1 L were dropped at different sites on each copolymer varnish, and the measured values of the contact angle were averaged. A microscope was used to observe the droplets profile. Before the analysis, the varnishes were dried during 1 week. Confocal laser scanning microscopy (CLSM): The water localization and the development of the biofilm were observed with a microscope Leica TCS-SP2 and an immersion 100× objective. To follow the development of microorganisms on the paints, the bacteria were stained with Syto 9 green fluorescent dye. Microalgae need no stain, they are autofluorescent. Then paint was washed with artificial seawater (ASW). Then the dry paint and microalgae were excited with the 633 nm line of Helium/Neon laser, and emission was collected from 650 to 750 nm. Bacteria were excited with 488 nm line of argon, and emission was collected at 500–550 nm. To visualize water in the coatings, fluorescein was added in water (25 g L−1 ). In this case, the same emission and excitation than for bacteria was used on the dry coating. Images were processed using Leica Confocal software. The amounts of microorganism on the surface were calculated using the ImageJ software. Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDX): To observe the paint surface and to obtain a
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Table 2 Results of the syntheses. Copolymers
Theoretical molar ratioa PCL-b-PDMS-b-PCL (%)
Molar ratio PCL/PDMS calculate by 1 H NMRb (%)
PCL 50 PCL 66 PCL 90
25-b-50-b-25 33-b-33-b-33 45-b-10-b-45
49/51 63/37 89/11
a b c
Theoretical molar masses (g mol−1 ) 5700 9100 33,000
Molar masses by 1 H NMR (g mol−1 ) 5600 8300 30,700
Molar masses by GPC (g mol−1 )c 9200 12,000 35,200
ÐM 1.22 1.28 1.35
Calculated with the starting molecular ratio of -CL and PDMS. Calculated with the relative intensities of the peak at 0.03–0.09 ppm and at 4.04–4.07 ppm. In THF as eluent and with polystyrene standard.
map of the paint components during the immersion, the paints were observed with an electron microscope JEOL 6460 LV equipped with an Oxford Inca 300 X-Ray microanalysis. To study the paint thickness, paint samples have been incorporated into a lowviscosity epoxy resin blend. Resin was poured into a ring form cup then was cured at room temperature for 48 h. They have been polished by a series of grinding (silicone carbide grinding paper P320–P1200) with water as lubricant. Then the polishing is performed progressively finer abrasives with two grades of diamond polishing grit suspension (9 m and 3 m). Then the polished specimens need to be metalized with carbon to obtain a conductive material. To study the surface, the dry paint was directly metalized. For all investigations, the beam energy was 20 kV or 13 kV in order to obtain the excitation of the element. To quantify elements present in the paints, INCA software was used. Nuclear magnetic resonance of proton (1H NMR): NMR 1 H spectra of the copolymers were obtained on a Bruker Avance 500 spectrometer at 500 MHz. The spectrum was taken in deuterated chloroform at 30 ◦ C. NMR 1 H (ppm): 0.06 [Si(CH3 )2 , 6H], 1.35 [CO(CH2 )2 CH2 (CH2 )2 O, 2H], 1.65 [COCH2 CH2 CH2 CH2 CH2 O, 4H], 2.3 [COCH2 , 2H], 4.05 [OCH2 , 2H]. The composition of copolymers was calculated from the ratios of absorbance at 0.06 ppm and 4.05 ppm. Fourier transform infrared spectroscopy by attenuated total reflectance (ATR-FTIR): IR spectra were performed on a Bruker Tensor 27 with a Harrick MVP 2 Series cell. The samples have been analyzed without preparation. Spectra were analyzed with the OPUS software. FTIR (cm−1 ): 2943 (C H, methylene of PCL), 1725 (>C O, ester of PCL); 800 (Si C, methyl of PDMS), 1097 (Si O, main chain of PDMS).
5. Results and discussion 5.1. Synthesis and characterization of binders In the present work, the ring opening polymerization of caprolactone was carried out with PDMS as initiator to synthesize a triblock copolymer and to obtain a binder for which the properties of both homopolymers should be present. In theory, the externals PCL blocks will be biodegradable and the PDMS core block will give surface properties to the material. Three copolymers were prepared to study the influence of the composition on their properties. These syntheses were carried out at 120 ◦ C for 48 h to obtain a complete
conversion of the monomer. The structure of the copolymers synthesized was determined by ATR-FTIR, GPC and 1 H NMR. The FTIR and 1 H NMR spectra exhibited all the relevant peaks of the two components allow to confirm the structural features of both polymers. Furthermore, the presence of a monomodal peak on the GPC proved the bond between the PCL and PDMS. For the purpose of the work, three different copolymers were prepared by varying their composition and molar masses. 1 H NMR spectroscopy was used to determine the molar mass and GPC for the evaluation of the polydispersity. As shown in Table 2, all the copolymers were synthesized with expected composition and molar mass. The difference between the molar mass obtained by GPC and RMN 1 H derives from the use of polystyrene standard for GPC analysis. After their characterization, the copolymers are used as binders for the preparation of antifouling paints. The components (Table 1) were added under a vigorous agitation and applied on different substrates (polycarbonate or polyester composite) to enable the evaluation of their antifouling performance.
5.2. Antifouling properties The evaluation of antifouling activity of coatings relies on in situ tests. Indeed, lab tests based on specific organisms cultivated in controlled conditions provide interesting data but remain incomplete and need a validation in natural conditions. For these reasons, the antifouling activity of prepared paints is evaluated by immersion of test panels in the Atlantic Ocean (Lorient harbour). The site was previously calibrated; the seasonal fluctuations of seawater parameters (pH, temperature, composition) are known. Two different methods are used to observe the adhesion and the development of marine organisms on the paints. The aim is to evaluate the antifouling activity at two main steps of biofouling formation: (i) the development of the microfouling and (ii) the proliferation of the macrofouling. Indeed, the settlement of biofouling on immersed surfaces in seawater is generally described as a phenomenon in stages: the conditioning of the substrate by ions, proteins and carbohydrates present in the surrounding water, reversible and irreversible adhesion of bacteria, settlement of microorganisms (microalgae for example) and finally the proliferation of multicellular species (algae, barnacles, mussels) (Fig. 2). The confocal laser scanning microscopy (CLSM) is a powerful tool to visualize microorganisms
Fig. 2. Development processes of marine fouling [23]. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
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Fig. 3. Development of biofilm observed on the paints surface by SCLSM during the in situ immersion. (*) Biofilm too thick: impossible to visualize. (**) Technical problem.
without further preparations that could damage the sample. The evaluation of the macrofouling colonization is done following a preestablished protocol referencing the percent of coverage and the species of organisms. The quotation takes into account the impact of the colonizing species on the ship performances [24]. Furthermore, in accordance with the published studies about antifouling evaluation the prepared paints are evaluated by comparison with an efficient commercial paint based on the same biocides [25]. The paint reference is a controlled erodible coating. 5.2.1. Development of microbiofouling The CLSM is an informative technique about the marine microfouling. Indeed, it gives data about the density of the biofilm, the architecture of the microorganisms colonies and information about marine species [26]; with different fluorescent stains, bacteria are
marked in green and microalgae in red as shown in Fig. 3. For the purpose of the study, the development of biofilm was studied during 24 weeks in summer, until all the paints surface are completely covered by microorganisms. Fig. 3 presents micrographs obtained for the hybrid paints and the commercial paint. The coating based on copolymer PCL 90 has the lowest activity against microfouling; after 8 weeks its surface is completely coated by microorganisms. After this period, the microfouling is too thick to observe the paint surface. The copolymer properties could explain this result. The other hybrid paints, PCL 50 and PCL 66, give interesting results, the antifouling activity is closed to the commercial paint. Until the 3rd week, the biofilm development is progressive; bacteria (in green on the pictures) and microalgae (in red) colonize the paints surface as explained by Fig. 2. The six following weeks a cleaning of the surface was
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Fig. 4. Development of macrofilm observed on the paints surface during the in situ immersion.
observed. Many causes could explain this phenomenon: an important release of biocides or a mechanical erosion of the surface for example. Then surfaces were colonized anew. After 24 weeks of immersion, the sole difference observed between the hybrids paints (PCL 50 and PCL 66) and the commercial paint (Ref) was the majority species colonizing the surface. It seems that the binder of hybrid paints prevents the bacteria adhesion. Otherwise, the amount of the microorganisms on the surface was very close, 80% for the reference and approximately 70% for the hybrid coatings. 5.2.2. Development of macrofouling The visual inspection of test panel allows the observation of macrofouling development. As shown in Fig. 4, the surface without any protection is completely colonized by algae after 10 weeks confirming the relevance of the selected site of immersion. The
comparison between hybrid paints and the commercial reference show that their antifouling activity is lower in the test conditions. The comparison between hybrids paint and the commercial paint show that the antifouling activity is lower in the test conditions. After 24 weeks in seawater, some algae are present on the hybrids paints surface while the reference keep being clean. The experimental condition could explain this result. Hybrid paints are supposed to combine two modes of action: the biocides release obtained by the hydration, degradation and erosion of the binder (PCL blocks) and the fouling release get via a pronounced hydrophobicity (PDMS blocks). Static conditions of immersion screen the surface properties of hybrid paints that need a hydrodynamic stress and so focalize the evaluation on PCL dependant parameter: the erosion, the degradation and the water diffusion.
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Table 3 Influence of the copolymer ratio on the crystallinity and on the topography surface of paints. Paint
PCL 50
PCL 66
PCL 90
PCL/PDMS ratio
50/50
66/33
90/10
15.9
32.9
42.8
Copolymer crystallinity (%)
Paint surface
For PCL 90, the major development of microfouling explains the quick and important colonization by algae and other macroorganisms. As briefly discussed previously, the antifouling activity has many origins: the biocide release, the erosion and the surface properties such as elasticity or hydrophobicity. Their respective contributions remain unclear in available technologies and need to be studied for hybrid systems. Of course, binders are a key parameter to control these characteristics. 5.3. Study of parameters involved in the antifouling activity The major properties of the paints come from the binder [27], so the triblock copolymers properties are mainly responsible of the antifouling activity. In this study, it is decided to focus investigation on the properties brought by the polyester blocks like the crystallinity, the hydration, the degradation, the erosion and the biocide release. Nevertheless, to evaluate the impact of silicone block, the hydrophobic surface properties have also been measured. These
properties have been studied in distilled water to have a quicker diffusion of water in the paint [28]. 5.3.1. Crystallinity The crystallinity is a key parameter in the study of polymer properties. In the case of antifouling coatings, it is suspected to impact the mechanical properties (elasticity), the additive and fillers dispersion and coating stability (pot life) and finally their hydration, degradation and release abilities [29,30]. The results obtained using Eq. (1) (Table 3) show a decrease of the crystallinity caused by the PDMS block. The copolymer PCL 90 with the lowest ratio of PDMS has the highest crystallinity (42.8%). Moreover, the SEM pictures of coatings surfaces before immersion show a clear difference of their topography. The more crystalline polymers, PCL 90 and PCL 66, lead to coating with some large cracks probably caused by a lack of elasticity during the drying. These observations explain the poor antifouling properties observed during the immersion for the paint PCL 90. The cracks are known to promote the settlement and the development of fouling
Fig. 5. Water observation in the thickness of paints by CLSM. Green (paint), red (fluorescein) and yellow (paint and fluorescein). (For interpretation of the references to color in the figure legend, the reader is referred to the web version of this article.)
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Fig. 6. Quantity of water in the copolymer and in his paint measured by Karl–Fisher coulometer. (I) PCL 50 and (II) PCL 66.
by providing a shelter to surrounding aggressions. The SEM pictures obtained after one week of immersion show an important settlement in the cracks. For the next properties only the PCL 66 and PCL 50 paints were studied, the paint PCL 90 does not meet the basic requirements. 5.3.2. Water absorption The water absorption is the one of the first event occurring after immersion of polymers. Moreover, the hydration has an influence on paint erosion and on biocides release [31] for commercial available paints. Two methods were used to study the water absorption of hybrid paints. CLSM allows to visualize water in the coating to determine whether the hydration is heterogeneous or not. The Karl–Fisher coulometer (KF) allows to quantify water absorbed by the paint. The observations by CLSM imply fluorescence. For this purpose, fluorescein was added in water. Two different behaviours were observed on the micrographs obtained by CLSM (Fig. 5). The paint PCL 66 shows progressive water absorption during the first month. After one day, fluorescein (in yellow) remains located only at the surface of the coating. Then after, hydration goes on and the stain is finally present in the whole thickness of the film after 35 days. Inversely, for the paint PCL 50, fluorescein is observed in the film as early as the first day of immersion. This difference could be explained by polymer properties and more precisely by their crystallinity that is lower for PCL 50. Progressive water absorption
is searched because it promotes a controlled release of biocides and erosion. Comparatively to the commercial reference paint, the hydration of hybrid coatings is faster and could lead to a reduce time life. The quantitative study carried out by KF enables to establish kinetics curves of hydration (Fig. 6). For both hybrid paints, hydration is slower in paints than in copolymers. Saturation state requires three times longer to be achieved. For example, the film of PCL 66 is totally hydrated after 20 days of immersion whereas the corresponding paint requires about 60 days. This result confirms the importance of additives and fillers in the behaviour of coatings during immersion. Their influence on the hydration rate is changing following the binder. For PCL 50, the saturated rate is approximately similar for the copolymer film and the paint, respectively 25% and 27%. While for PCL 66, the paint absorbs less water (23%) than the copolymer film (29%). This difference could be explained by crystallinity and the macromolecular chains organization that are probably different. 5.3.3. Degradation and chemicals release The paint degradation comes mainly from the binder degradation and from the release of components (biocides, hydrophilic fillers). Moreover some studies have shown that the binder degradation promotes the biocides releases in many cases such as self-polishing copolymers [5,27]. In the case of hybrid paints, it
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Fig. 7. Copolymers degradation in the paint follows by GPC.
is interesting to investigate this potential link. Two studies were carried out to evaluate the degradation of the copolymer by gel permeation chromatography (GPC) and to measure chemical release by energy dispersive X-ray spectrometry (EDX). The kinetics curves of degradation for both copolymers PCL 50 and PCL 66 are close (Fig. 7). The paint PCL 50s has a full hydration of 27% and for the paint PCL 66s it is 23%. Nevertheless the paint with the slower kinetic of hydration needs more time to reach the maximum of binder degradation. The paint PCL 66s is totally hydrated in 150 days and the maximum of degradation is reached after 300 days whereas the hydration and the degradation for PCL 50 go faster, respectively 125 and 250 days are needed to reach saturation.
This result proves a link between the hydration of the coating and the degradation of the copolymer, as previously observed for this copolymer [21] and for other antifouling systems [13]. The molecules release is quantified by EDX. The surface and thickness coating analysis was focused on two biocides, zinc pyrithione (zinc) and dichlofluanid (chlorine), and a hydrophilic filler, calcium carbonate (calcium) (Fig. 8). Both paints have similar release kinetics. The amount of each element decreases quickly. In 30 days, the major part was released. The remaining zinc that is observed in the coating comes from the zinc oxide used as pigment and filler. This quick release may explain the surface cleaning observed during the development of microfouling after one month. Moreover, this analysis technique allowed to prove the absence of
Fig. 8. Biocides and hydrophilic components release quantified by EDX. (a ) Amount of each element normalized by titanium amount present in the surface and thickness of coating. Titanium is not released.
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Fig. 9. Contact angle measured with water on the binders surface during their immersion in distilled water.
leaching of copper (I) thiocyanate (copper) and stannous octanoate (tin) (data not shown). This result is important from an environmental point of view. These results underline that the degradation of hybrid polymers in not discriminating factor for biocides release. The quick release is essentially due to the quick hydration revealed by CLSM. Biocides are hydrophilic and after one month water was found in all the thickness of the coating. 5.3.4. Erosion During the immersion in natural site, an important difference for erosion is observed. The amount of paint removed by rubbing with a cotton ball on the surface is roughly estimated. After 24 weeks, the commercial paint keeps being erodible with constant erosion kinetics all over the period. Inversely, the hybrid paints PCL 50 and PCL 66 have shown a stop of the erosion from the fourth and the fifth month of immersion respectively. Globally the paint PCL 66 is more erodible than PCL 50. The end of erosion corresponds with the increase of algae on their surface. The erosion promotes a cleaning of the surface by the release of biocides (during the first month for hybrid paints), but also by a mechanical action due to a surface degradation with the release of hydrophilic fillers and binder. Moreover, the water saturation evaluate in coatings corresponds approximately with the end of erosion observed during the in situ test. Even if the hydration is quicker in distilled water than in seawater, these results seem confirm that to obtain constant erosion during the paint lifetime, the hybrid paint should be able to absorb water continuously. However the results also show that erosion and degradation of hybrid polymers are not directly linked. The degradation of the copolymers goes on longtime after the end of erosion. 5.3.5. Surface properties To check the hybrid properties of binder due to the PDMS block, the surface hydrophobicity is monitored during immersion. The measurement of water contact angle of hybrid copolymers PCL 50s and 66s have been compared to a reference of poly(-caprolactone) homopolymer (Mn = 9000 g mol−1 ). Fig. 9 illustrates the results and shows a significant influence of PDMS block on surface properties. Both hybrid binders have a more hydrophobic (water contact angle >110◦ ) surface than the PCL reference (water contact angle <100◦ ). In a first stage (during the 100 first days of immersion), the hydrophobic properties of
both hybrid polymers decrease like for the reference polymer (PCL 9000). For the homopolymer PCL 9000, the decrease is observed while the water diffusion and polymer degradation is the most important. When the degradation decreases and the coating has reached the maximum of hydration, the contact angle of the reference remains constant at about 80◦ . The binders PCL 50 and PCL 66 have another behaviour. Their surface becomes more hydrophobic: their water contact angle increases to reach a value of 110◦ . The surface properties of PDMS become progressively predominant. So these results confirm the interest of this kind of binder that combines controlled erosion, biocides release and hydrophobicity during immersion. Furthermore, although hydrophobicity is a key parameter in the fouling release strategy, this parameter is not taken into account in the antifouling tests previously realized. Under hydrodynamic stress, the performances of hybrid paints are supposed to be higher. 6. Conclusion This work has shown the interest to develop hybrid binders. It is could be a new way to have antifouling paints more efficient and more environmental. The use of a copolymer with poly(caprolactone) and poly(dimethylsiloxane) as binder has done a paint that associates hydrophobicity and biodegradable properties. The corresponding paints are able to prevent fouling settlement and proliferation by (i) erosion, (ii) biocides release and (iii) high and constant hydrophobicity during one month of immersion. Among the parameters involved in the antifouling efficiency crystallinity, hydration, degradation, erosion and surface properties have been studied. It seems that crystallinity and hydration are key parameters to control the release of bioactive molecules in hybrid systems. But no link was observed between degradation and erosion. Some works is required on the formulation to have a better control on the biocides release and reduce hydration kinetics in order to increase the paint life. Furthermore, it should be interesting to test hybrid coatings in dynamic conditions to evaluate the PDMS contribution to antifouling activity. Acknowledgements This work was supported by the Region Bretagne (France) and PPG Coatings Europe. We sincerely thank Evonik Tego Chemie GmbH Company for the kind gift of the functional silicone monomer (Tegomer® H-Si 2311).
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References [1] C. Compère, M.N. Bellon-Fontaine, P. Bertrand, D. Costa, P. Marcus, C. Poleunis, C.M. Pradier, B. Rondot, M.G. Walls, Kinetics of conditioning layer formation on stainless steel immersed in seawater, Biofouling 17 (2001) 129–145. [2] M.P. Schultz, J.A. Bendick, E.R. Holm, W. Hertel, Economic impact of biofouling on a naval surface ship, Biofouling 27 (2011) 87–983. [3] D.M. Yebra, S. Kiil, K. Dam-Johansen, Antifouling technology – past, present and future steps towards efficient and environmentally friendly antifouling coatings, Prog. Org. Coat. 50 (2004) 75–104. [4] S. De Nancay, Nav. Ports Chantiers 39 (1989) 238. [5] M. Lejars, A. Margaillan, C. Bressy, Fouling release coatings: a nontoxic alternative to biocidal antifouling coatings, Chem. Rev. 112 (2012) 4347–4390. [6] K.A. Dafforn, J.A. Lewis, E.L. Jonhston, Antifouling strategies: history and regulation, ecological impacts and mitigation, Mar. Pollut. Bull. 62 (2011) 453–465. [7] IMO, International Convention on the Control of Harmful Antifouling Systems on Ships, International Maritime Organisation, London, 2001. [8] C.L. Alzieu, J. Sanjuan, J.P. Deltreil, M. Borel, Tin contamination in Arcachon bay: effect on oyster shell anomalies, Mar. Pollut. Bull. 17 (1986) 494–498. ¨ I. Linossier, K. Reh´ıel, Advances in Marine [9] C. Bressy, A. Margaillan, F. Fay, Antifouling Coatings and Technologies, Woodshead Publishing, Cambridge, UK, 2009, pp. 445. [10] C.M. Magin, S.P. Cooper, A.B. Brennan, Non-toxic antifouling strategies, Mater. Today 13 (4) (2010) 36–44. [11] M.P. Schultz, C.J. Kavanagh, G.W. Swain, Hydrodynamic forces on barnacles: implications on detachment from fouling-release surfaces, Biofouling 13 (1999) 323–335. [12] R.F. Brady, Fouling-release coatings for warships, Def. Sci. J. 55 (2005) 75–81. ¨ I. Linossier, V. Langlois, E. Renard, K. Vallée-Réhel, Degradation and [13] F. Fay, controlled release behavior of -caprolactone copolymers in biodegradable antifouling coatings, Biomacromolecules 7 (2006) 851–857. [14] J.M. Pelto, S. Virtanen, T. Munter, J. Larismaa, S. Jämsa, J. Nikkola, Encapsulation of 3-iodo-2-propynyl-N-butylcarbamate (IPBC) in polystyrenepolycaprolactone (PS/PCL) blends, J. Microencapsul. 31 (2014) 415–421. ˛ [15] M. Rutkowska, M. Jastrzebska, H. Janik, Biodegradation of polycaprolactone in sea water, React. Funct. Polym. 38 (1998) 27–30. [16] S.C. Woodward, P.S. Brewer, F. Moatamed, A. Schindler, C.G. Pitt, The intracellular degradation of poly-epsilon-caprolactone, J. Biomed. Mater. Res. 19 (1985) 437–444.
19
[17] R.A. Gross, B. Kalra, Biodegradable polymers for the environment, Science 297 (2002) 803–807. [18] K.J. Wynne, G.W. Swain, R.B. Fox, S. Bullock, J. Uilk, Two silicone nontoxic fouling release coatings: hydrosilation cured PDMS and CaCO3 filled, ethoxysiloxane cured RTV11, Biofouling 16 (2000) 277–288. [19] D. Wendt, G. Kowalke, J. Kim, I. Singer, Factors that influence elastomeric coating performance: the effect of coating thickness on basal plate morphology, growth and critical removal stress of the barnacle Balanus amphitrite, Biofouling 22 (2006) 1–9. [20] U. Buchholz, R. Mulhaupt, Branched polysiloxane block copolymers as epoxy toughening agents, ACS Polym. Prepr. 33 (1992) 205–206. ¨ K. Réhel, I. Linossier, Control of hydration and degradation [21] F. Azemar, F. Fay, properties of triblock copolymers polycaprolactone-b-polydimethylsiloxaneb-polycaprolactone, J. Appl. Polym. Sci. (2014) 2014. [22] M.F. Koenig, S.J. Huang, Biodegradable blends and composites of polycaprolactone and starch derivatives, Polymer 36 (1995) 1877–1882. [23] D. Haras, Biofilms and deteriorations of materials: from analysis of the phenomenon to prevention strategies, Mater. Tech. 93 (2006) s.27–s.41. [24] C. Bressy, J.F. Briand, C. Compère, K. Réhel, Efficacy testing of biocides and biocidal coatings, in: S. Dobretsov, J.C. Thomason, N.D. Wlliams (Eds.), Biofouling Methods, John Wiley & Sons, Oxford, 2014 (Chapter 12). [25] NF T 34-552, Peintures et vernis – Systèmes de peintures pour la protection des ouvrages en acier – Essai d’immersion au radeau en eau de mer vive, 1996. [26] D.E. Caldwaell, D.R. Korber, J.R. Lawrence, Confocal laser microscopy and digital image analysis in microbial ecology, Adv. Microb. Ecol. 12 (1992) 1–67. [27] E. Almeida, T.C. Diamantino, O. De Sousa, Marine paints: the particular case of antifouling paints, Prog. Org. Coat. 59 (2007) 2–20. [28] P. Castaing, Vieillisement des matériaux composites verre-polyester en milieu marin: délaminage d’origine osmotique (Thesis), 1992, pp. 34. [29] J.Z. Bei, J.M. Li, J.C. Le, S.G. Wang, Polycaprolactone-poly(ethylene-glycol) block copolymer. IV. Biodegradation behaviour in vitro and in vivo, Polym. Adv. Technol. 8 (1997) 693–696. [30] M.N. Kim, K.H. Kim, H.H. Jin, J.K. Park, J.S. Yoon, Biodegradability of ethyl and n-octyl branched poly(ethylene adipate) and poly(butylene succinate), Eur. Polym. J. 37 (2001) 1843–1847. ¨ I. Linossier, J.J. Peron, V. Langlois, K. Vallée-Réhel, Antifouling [31] F. Fay, activity of marine paints: study of erosion, Prog. Org. Coat. 60 (2007) 194–206.
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Development of hybrid antifouling paints ¨ Karine Réhel, Isabelle Linossier Fabrice Azemar ∗ , Fabienne Fay, Laboratoire de Biotechnologie et Chimie Marines, Univ. Bretagne-Sud, EA 3884, LBCM, IUEM, F-56100 Lorient, France
a r t i c l e
i n f o
Article history: Received 3 October 2014 Received in revised form 8 March 2015 Accepted 13 April 2015 Keywords: Antifouling Hybrid paints Poly(-caprolactone) Poly(dimethylsiloxane) Biofouling Binder
a b s t r a c t The objectives of this study are to measure the efficiency of a hybrid system for antifouling paint and to observe the influence of the physico-chemical properties of the binder on the antifouling activity. Poly(caprolactone) (PCL) and poly(dimethylsiloxane) (PDMS) homopolymers are already used as binder for different antifouling strategies. The use of the block copolymer should allow to mix the properties of erosion and hydrophobicity to obtain a more efficient paint with a reduced environmental impact. Paints containing triblock copolymer were immersed in seawater in Lorient harbour to evaluate their antifouling activities. The water absorption, the degradation and the surface properties of the copolymer were measured in distilled water to understand the influence of the binder characteristics in the resulting antifouling properties. The hybrid paints have shown efficiency close to a commercial paint during their immersion in situ in spite of inadequate static conditions of test. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Antifouling paints are the main system used against the development of organisms (bacteria, algae, mollusc, etc.) on immerged surface and the most studied. The colonization by organism and their secretion form the biofilm [1]. The biofouling development on the surfaces is an economic and environmental problem [2]. It increases the surface roughness, the material degradation and the fuel consumption of ships. Moreover, it should be responsible for the introduction of invasive species [3]. Since the 19th century and the use of first antifouling paints [4], lot of researches have been done to increase efficiency against the biofouling and the lifetime [5]. However, these improvements have been done to the detriment of the environment. The more known example is the introduction of tributyltin-based compounds (TBT) as biocide in paints formulation to obtain cheap paints with an efficiency of 5 years [6]. Unfortunately, its use has caused many troubles on marine species due to the accumulation and severe toxicity of TBT [7,8]. To substitute paints with TBT, two main techniques were used: the biocide release strategy based on the incorporation of natural biocides or synthetic biocides less harmful for the environment [9] and the fouling release strategy based on paints with physico-chemical surface properties to limit durable colonization
∗ Corresponding author. Tel.: +33 681758801. E-mail address: [email protected] (F. Azemar). http://dx.doi.org/10.1016/j.porgcoat.2015.04.007 0300-9440/© 2015 Elsevier B.V. All rights reserved.
by organisms [10]. But, the development of new biocides and new paints have greatly increased the paints price without increase their efficiency. Moreover, both kinds of paints have shown other disadvantages. For the coatings with biocides, the new European and international regulations on chemical products will limit their uses for environmental application. Concerning paints with surface properties, it is their inefficiency during static periods is problematic [11,12]. Indeed the biofilm is removed by hydrodynamic stress like navigation for ships or a mechanical cleaning. In the aim to develop a new kind of more environmental antifouling paint, two polymers seem appropriate. A polyester, the poly(-caprolactone) (PCL) which has been already used as binder for antifouling paints [13,14]. Its biodegradable properties [15,16] are well known and allow to release biocide. Furthermore, polyesters seem to be a perfect candidate for a new generation of antifouling paints, they are already used for environmental application [17] and they are cheap. Another interesting polymer already used for antifouling paints is a silicone, the poly(dimethylsiloxane) (PDMS). It is the most found silicone in commercial application and it has done good results as binder in paints with surface properties [18,19]. Moreover, PDMS homopolymers are already used by some companies (Chugoku Marine Paints, PPG, etc.) for their antifouling paints [5]. Buchholz and Mülhaupt have already synthesized a triblock copolymer containing these two polymers [20]. Moreover the block copolymer poly(-caprolactone)-blockpoly(dimethylsiloxane)-block-poly(-caprolactone) (PCL-b-PDMSb-PCL) obtained by this way have shown interesting properties of hydration and degradation in a previous work of our group [21].
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Fig. 1. Synthesis of the binder by ring opening polymerization of -caprolactone.
In the present work, the linear triblock copolymer was incorporate as binder in antifouling paint to measure the efficiency of this system in seawater and to evaluate the influence of polymer properties on the antifouling activity. Furthermore, this work have to shown the interest to develop a hybrid system who should allow to limit the quantity of biocide in the paints and to have a high efficiency in all conditions due to the properties of both polymers.
2. Experimental 2.1. Materials The monomer of -caprolactone (CL) and the catalyst stannous octanoate (Sn(Oct)2 ) were purchased from Aldrich Co. The polydimethylsiloxane Tegomer® H-Si 2311 (Mn ≈2300 g mol−1 ) was kindly supplied by Evonik Industries, Germany. All the products were used as received.
2.2. Binders synthesis Binders are triblock copolymers with poly(-caprolactone) external blocks and a polydimethylsiloxane core block. The copolymers were synthesized by ring opening polymerization of -CL in the presence of PDMS with Sn(Oct)2 as catalyst (Fig. 1). The copolymer composition was varied by changing the feed ratio of the reactants [CL]/[PDMS] (270:1; 60:1; 30:1 n/n, respectively) to study the effect of the composition on the antifouling properties. The same procedure than described in a precedent work was used [21].
3. Paint preparation To evaluate the physico-chemical and antifouling properties of the binders, paints were formulated with the three copolymers. All the components presented in Table 1 were dispersed under vigorous agitation during 30 min. Then the paints were applied on polycarbonate plates for the studies of the physico-chemical properties and on glass–polyester composite plates for the evaluation of antifouling properties.
Table 1 Components of the paints. Solvents
Xylene MIBK
Binder
PCL-b-PDMS-b-PCL
Fillers
TiO2 ZnO CaCO3
Additives
Dichlofluanide Zinc pyrithione Copper thiocyanate Polyamide waxes
4. Measurement Differential scanning calorimetry (DSC): Calorimetric data were obtained using a Mettler-Toledo DSC 822 (Mettler Toledo, Viroflay, France). The calibration was done with indium and zinc. Aluminium pans were used and the sample mass was approximately 10 mg. The samples were first melted to 100 ◦ C (1st run/20 ◦ C min−1 ) and kept at this temperature for 2 min, then cooled to −100 ◦ C and heated up again to 200 ◦ C (2nd run/20 ◦ C min−1 ). Temperature and heat of phase transitions were determined respectively from the maxima as well as crystallization areas and melting peaks. The degree of crystallinity was estimated using Eq. (1) where Hm is the measured melting enthalpy of the polymeric part of the sample and H100% the equilibrium melting enthalpy of 100% crystalline PCL taken equal to 139.3 J g−1 [22]. Xc =
Hm × M%PCL H100%
(1)
Karl–Fisher coulometer (KF): The quantity of water in the paints during the immersion were determined by a Metrohm KF 737 equipped with a Metrohm Oven KF 707 (T = 150 ◦ C) which was used under a nitrogen flow of 200 mL min−1 . The reactant was HydranalCoulomat AG. To measure only the water in the film, the samples were wiped before inserted in the coulometer. Gel permeation chromatography (GPC): GPC chromatograms were measured by a Merck pump L-7110, two columns PLgel (Mixed-E, 3 m and Mixed-D, 5 m) from polymer laboratories, and a Sedex DEDL detector. THF was used as eluent at a flow rate of 1 mL min−1 , and the injection volume was 20 L. The calibration was done with polystyrene standards with molar masses included between 1300 and 400,000 g mol−1 (Easical PS-2) from Agilent Technologies. Before the injection, the dry paints were centrifuged at 5000 rpm during 10 min at 20 ◦ C to eliminate water. Then the supernatant was filtered with PTFE filter (0.45 m). Contact angle measurement: Measurements were taken at room temperature with a contact angle metre Digidrop GBX. Ten droplets of 1 L were dropped at different sites on each copolymer varnish, and the measured values of the contact angle were averaged. A microscope was used to observe the droplets profile. Before the analysis, the varnishes were dried during 1 week. Confocal laser scanning microscopy (CLSM): The water localization and the development of the biofilm were observed with a microscope Leica TCS-SP2 and an immersion 100× objective. To follow the development of microorganisms on the paints, the bacteria were stained with Syto 9 green fluorescent dye. Microalgae need no stain, they are autofluorescent. Then paint was washed with artificial seawater (ASW). Then the dry paint and microalgae were excited with the 633 nm line of Helium/Neon laser, and emission was collected from 650 to 750 nm. Bacteria were excited with 488 nm line of argon, and emission was collected at 500–550 nm. To visualize water in the coatings, fluorescein was added in water (25 g L−1 ). In this case, the same emission and excitation than for bacteria was used on the dry coating. Images were processed using Leica Confocal software. The amounts of microorganism on the surface were calculated using the ImageJ software. Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDX): To observe the paint surface and to obtain a
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Table 2 Results of the syntheses. Copolymers
Theoretical molar ratioa PCL-b-PDMS-b-PCL (%)
Molar ratio PCL/PDMS calculate by 1 H NMRb (%)
PCL 50 PCL 66 PCL 90
25-b-50-b-25 33-b-33-b-33 45-b-10-b-45
49/51 63/37 89/11
a b c
Theoretical molar masses (g mol−1 ) 5700 9100 33,000
Molar masses by 1 H NMR (g mol−1 ) 5600 8300 30,700
Molar masses by GPC (g mol−1 )c 9200 12,000 35,200
ÐM 1.22 1.28 1.35
Calculated with the starting molecular ratio of -CL and PDMS. Calculated with the relative intensities of the peak at 0.03–0.09 ppm and at 4.04–4.07 ppm. In THF as eluent and with polystyrene standard.
map of the paint components during the immersion, the paints were observed with an electron microscope JEOL 6460 LV equipped with an Oxford Inca 300 X-Ray microanalysis. To study the paint thickness, paint samples have been incorporated into a lowviscosity epoxy resin blend. Resin was poured into a ring form cup then was cured at room temperature for 48 h. They have been polished by a series of grinding (silicone carbide grinding paper P320–P1200) with water as lubricant. Then the polishing is performed progressively finer abrasives with two grades of diamond polishing grit suspension (9 m and 3 m). Then the polished specimens need to be metalized with carbon to obtain a conductive material. To study the surface, the dry paint was directly metalized. For all investigations, the beam energy was 20 kV or 13 kV in order to obtain the excitation of the element. To quantify elements present in the paints, INCA software was used. Nuclear magnetic resonance of proton (1H NMR): NMR 1 H spectra of the copolymers were obtained on a Bruker Avance 500 spectrometer at 500 MHz. The spectrum was taken in deuterated chloroform at 30 ◦ C. NMR 1 H (ppm): 0.06 [Si(CH3 )2 , 6H], 1.35 [CO(CH2 )2 CH2 (CH2 )2 O, 2H], 1.65 [COCH2 CH2 CH2 CH2 CH2 O, 4H], 2.3 [COCH2 , 2H], 4.05 [OCH2 , 2H]. The composition of copolymers was calculated from the ratios of absorbance at 0.06 ppm and 4.05 ppm. Fourier transform infrared spectroscopy by attenuated total reflectance (ATR-FTIR): IR spectra were performed on a Bruker Tensor 27 with a Harrick MVP 2 Series cell. The samples have been analyzed without preparation. Spectra were analyzed with the OPUS software. FTIR (cm−1 ): 2943 (C H, methylene of PCL), 1725 (>C O, ester of PCL); 800 (Si C, methyl of PDMS), 1097 (Si O, main chain of PDMS).
5. Results and discussion 5.1. Synthesis and characterization of binders In the present work, the ring opening polymerization of caprolactone was carried out with PDMS as initiator to synthesize a triblock copolymer and to obtain a binder for which the properties of both homopolymers should be present. In theory, the externals PCL blocks will be biodegradable and the PDMS core block will give surface properties to the material. Three copolymers were prepared to study the influence of the composition on their properties. These syntheses were carried out at 120 ◦ C for 48 h to obtain a complete
conversion of the monomer. The structure of the copolymers synthesized was determined by ATR-FTIR, GPC and 1 H NMR. The FTIR and 1 H NMR spectra exhibited all the relevant peaks of the two components allow to confirm the structural features of both polymers. Furthermore, the presence of a monomodal peak on the GPC proved the bond between the PCL and PDMS. For the purpose of the work, three different copolymers were prepared by varying their composition and molar masses. 1 H NMR spectroscopy was used to determine the molar mass and GPC for the evaluation of the polydispersity. As shown in Table 2, all the copolymers were synthesized with expected composition and molar mass. The difference between the molar mass obtained by GPC and RMN 1 H derives from the use of polystyrene standard for GPC analysis. After their characterization, the copolymers are used as binders for the preparation of antifouling paints. The components (Table 1) were added under a vigorous agitation and applied on different substrates (polycarbonate or polyester composite) to enable the evaluation of their antifouling performance.
5.2. Antifouling properties The evaluation of antifouling activity of coatings relies on in situ tests. Indeed, lab tests based on specific organisms cultivated in controlled conditions provide interesting data but remain incomplete and need a validation in natural conditions. For these reasons, the antifouling activity of prepared paints is evaluated by immersion of test panels in the Atlantic Ocean (Lorient harbour). The site was previously calibrated; the seasonal fluctuations of seawater parameters (pH, temperature, composition) are known. Two different methods are used to observe the adhesion and the development of marine organisms on the paints. The aim is to evaluate the antifouling activity at two main steps of biofouling formation: (i) the development of the microfouling and (ii) the proliferation of the macrofouling. Indeed, the settlement of biofouling on immersed surfaces in seawater is generally described as a phenomenon in stages: the conditioning of the substrate by ions, proteins and carbohydrates present in the surrounding water, reversible and irreversible adhesion of bacteria, settlement of microorganisms (microalgae for example) and finally the proliferation of multicellular species (algae, barnacles, mussels) (Fig. 2). The confocal laser scanning microscopy (CLSM) is a powerful tool to visualize microorganisms
Fig. 2. Development processes of marine fouling [23]. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
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Fig. 3. Development of biofilm observed on the paints surface by SCLSM during the in situ immersion. (*) Biofilm too thick: impossible to visualize. (**) Technical problem.
without further preparations that could damage the sample. The evaluation of the macrofouling colonization is done following a preestablished protocol referencing the percent of coverage and the species of organisms. The quotation takes into account the impact of the colonizing species on the ship performances [24]. Furthermore, in accordance with the published studies about antifouling evaluation the prepared paints are evaluated by comparison with an efficient commercial paint based on the same biocides [25]. The paint reference is a controlled erodible coating. 5.2.1. Development of microbiofouling The CLSM is an informative technique about the marine microfouling. Indeed, it gives data about the density of the biofilm, the architecture of the microorganisms colonies and information about marine species [26]; with different fluorescent stains, bacteria are
marked in green and microalgae in red as shown in Fig. 3. For the purpose of the study, the development of biofilm was studied during 24 weeks in summer, until all the paints surface are completely covered by microorganisms. Fig. 3 presents micrographs obtained for the hybrid paints and the commercial paint. The coating based on copolymer PCL 90 has the lowest activity against microfouling; after 8 weeks its surface is completely coated by microorganisms. After this period, the microfouling is too thick to observe the paint surface. The copolymer properties could explain this result. The other hybrid paints, PCL 50 and PCL 66, give interesting results, the antifouling activity is closed to the commercial paint. Until the 3rd week, the biofilm development is progressive; bacteria (in green on the pictures) and microalgae (in red) colonize the paints surface as explained by Fig. 2. The six following weeks a cleaning of the surface was
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Fig. 4. Development of macrofilm observed on the paints surface during the in situ immersion.
observed. Many causes could explain this phenomenon: an important release of biocides or a mechanical erosion of the surface for example. Then surfaces were colonized anew. After 24 weeks of immersion, the sole difference observed between the hybrids paints (PCL 50 and PCL 66) and the commercial paint (Ref) was the majority species colonizing the surface. It seems that the binder of hybrid paints prevents the bacteria adhesion. Otherwise, the amount of the microorganisms on the surface was very close, 80% for the reference and approximately 70% for the hybrid coatings. 5.2.2. Development of macrofouling The visual inspection of test panel allows the observation of macrofouling development. As shown in Fig. 4, the surface without any protection is completely colonized by algae after 10 weeks confirming the relevance of the selected site of immersion. The
comparison between hybrid paints and the commercial reference show that their antifouling activity is lower in the test conditions. The comparison between hybrids paint and the commercial paint show that the antifouling activity is lower in the test conditions. After 24 weeks in seawater, some algae are present on the hybrids paints surface while the reference keep being clean. The experimental condition could explain this result. Hybrid paints are supposed to combine two modes of action: the biocides release obtained by the hydration, degradation and erosion of the binder (PCL blocks) and the fouling release get via a pronounced hydrophobicity (PDMS blocks). Static conditions of immersion screen the surface properties of hybrid paints that need a hydrodynamic stress and so focalize the evaluation on PCL dependant parameter: the erosion, the degradation and the water diffusion.
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Table 3 Influence of the copolymer ratio on the crystallinity and on the topography surface of paints. Paint
PCL 50
PCL 66
PCL 90
PCL/PDMS ratio
50/50
66/33
90/10
15.9
32.9
42.8
Copolymer crystallinity (%)
Paint surface
For PCL 90, the major development of microfouling explains the quick and important colonization by algae and other macroorganisms. As briefly discussed previously, the antifouling activity has many origins: the biocide release, the erosion and the surface properties such as elasticity or hydrophobicity. Their respective contributions remain unclear in available technologies and need to be studied for hybrid systems. Of course, binders are a key parameter to control these characteristics. 5.3. Study of parameters involved in the antifouling activity The major properties of the paints come from the binder [27], so the triblock copolymers properties are mainly responsible of the antifouling activity. In this study, it is decided to focus investigation on the properties brought by the polyester blocks like the crystallinity, the hydration, the degradation, the erosion and the biocide release. Nevertheless, to evaluate the impact of silicone block, the hydrophobic surface properties have also been measured. These
properties have been studied in distilled water to have a quicker diffusion of water in the paint [28]. 5.3.1. Crystallinity The crystallinity is a key parameter in the study of polymer properties. In the case of antifouling coatings, it is suspected to impact the mechanical properties (elasticity), the additive and fillers dispersion and coating stability (pot life) and finally their hydration, degradation and release abilities [29,30]. The results obtained using Eq. (1) (Table 3) show a decrease of the crystallinity caused by the PDMS block. The copolymer PCL 90 with the lowest ratio of PDMS has the highest crystallinity (42.8%). Moreover, the SEM pictures of coatings surfaces before immersion show a clear difference of their topography. The more crystalline polymers, PCL 90 and PCL 66, lead to coating with some large cracks probably caused by a lack of elasticity during the drying. These observations explain the poor antifouling properties observed during the immersion for the paint PCL 90. The cracks are known to promote the settlement and the development of fouling
Fig. 5. Water observation in the thickness of paints by CLSM. Green (paint), red (fluorescein) and yellow (paint and fluorescein). (For interpretation of the references to color in the figure legend, the reader is referred to the web version of this article.)
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Fig. 6. Quantity of water in the copolymer and in his paint measured by Karl–Fisher coulometer. (I) PCL 50 and (II) PCL 66.
by providing a shelter to surrounding aggressions. The SEM pictures obtained after one week of immersion show an important settlement in the cracks. For the next properties only the PCL 66 and PCL 50 paints were studied, the paint PCL 90 does not meet the basic requirements. 5.3.2. Water absorption The water absorption is the one of the first event occurring after immersion of polymers. Moreover, the hydration has an influence on paint erosion and on biocides release [31] for commercial available paints. Two methods were used to study the water absorption of hybrid paints. CLSM allows to visualize water in the coating to determine whether the hydration is heterogeneous or not. The Karl–Fisher coulometer (KF) allows to quantify water absorbed by the paint. The observations by CLSM imply fluorescence. For this purpose, fluorescein was added in water. Two different behaviours were observed on the micrographs obtained by CLSM (Fig. 5). The paint PCL 66 shows progressive water absorption during the first month. After one day, fluorescein (in yellow) remains located only at the surface of the coating. Then after, hydration goes on and the stain is finally present in the whole thickness of the film after 35 days. Inversely, for the paint PCL 50, fluorescein is observed in the film as early as the first day of immersion. This difference could be explained by polymer properties and more precisely by their crystallinity that is lower for PCL 50. Progressive water absorption
is searched because it promotes a controlled release of biocides and erosion. Comparatively to the commercial reference paint, the hydration of hybrid coatings is faster and could lead to a reduce time life. The quantitative study carried out by KF enables to establish kinetics curves of hydration (Fig. 6). For both hybrid paints, hydration is slower in paints than in copolymers. Saturation state requires three times longer to be achieved. For example, the film of PCL 66 is totally hydrated after 20 days of immersion whereas the corresponding paint requires about 60 days. This result confirms the importance of additives and fillers in the behaviour of coatings during immersion. Their influence on the hydration rate is changing following the binder. For PCL 50, the saturated rate is approximately similar for the copolymer film and the paint, respectively 25% and 27%. While for PCL 66, the paint absorbs less water (23%) than the copolymer film (29%). This difference could be explained by crystallinity and the macromolecular chains organization that are probably different. 5.3.3. Degradation and chemicals release The paint degradation comes mainly from the binder degradation and from the release of components (biocides, hydrophilic fillers). Moreover some studies have shown that the binder degradation promotes the biocides releases in many cases such as self-polishing copolymers [5,27]. In the case of hybrid paints, it
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Fig. 7. Copolymers degradation in the paint follows by GPC.
is interesting to investigate this potential link. Two studies were carried out to evaluate the degradation of the copolymer by gel permeation chromatography (GPC) and to measure chemical release by energy dispersive X-ray spectrometry (EDX). The kinetics curves of degradation for both copolymers PCL 50 and PCL 66 are close (Fig. 7). The paint PCL 50s has a full hydration of 27% and for the paint PCL 66s it is 23%. Nevertheless the paint with the slower kinetic of hydration needs more time to reach the maximum of binder degradation. The paint PCL 66s is totally hydrated in 150 days and the maximum of degradation is reached after 300 days whereas the hydration and the degradation for PCL 50 go faster, respectively 125 and 250 days are needed to reach saturation.
This result proves a link between the hydration of the coating and the degradation of the copolymer, as previously observed for this copolymer [21] and for other antifouling systems [13]. The molecules release is quantified by EDX. The surface and thickness coating analysis was focused on two biocides, zinc pyrithione (zinc) and dichlofluanid (chlorine), and a hydrophilic filler, calcium carbonate (calcium) (Fig. 8). Both paints have similar release kinetics. The amount of each element decreases quickly. In 30 days, the major part was released. The remaining zinc that is observed in the coating comes from the zinc oxide used as pigment and filler. This quick release may explain the surface cleaning observed during the development of microfouling after one month. Moreover, this analysis technique allowed to prove the absence of
Fig. 8. Biocides and hydrophilic components release quantified by EDX. (a ) Amount of each element normalized by titanium amount present in the surface and thickness of coating. Titanium is not released.
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Fig. 9. Contact angle measured with water on the binders surface during their immersion in distilled water.
leaching of copper (I) thiocyanate (copper) and stannous octanoate (tin) (data not shown). This result is important from an environmental point of view. These results underline that the degradation of hybrid polymers in not discriminating factor for biocides release. The quick release is essentially due to the quick hydration revealed by CLSM. Biocides are hydrophilic and after one month water was found in all the thickness of the coating. 5.3.4. Erosion During the immersion in natural site, an important difference for erosion is observed. The amount of paint removed by rubbing with a cotton ball on the surface is roughly estimated. After 24 weeks, the commercial paint keeps being erodible with constant erosion kinetics all over the period. Inversely, the hybrid paints PCL 50 and PCL 66 have shown a stop of the erosion from the fourth and the fifth month of immersion respectively. Globally the paint PCL 66 is more erodible than PCL 50. The end of erosion corresponds with the increase of algae on their surface. The erosion promotes a cleaning of the surface by the release of biocides (during the first month for hybrid paints), but also by a mechanical action due to a surface degradation with the release of hydrophilic fillers and binder. Moreover, the water saturation evaluate in coatings corresponds approximately with the end of erosion observed during the in situ test. Even if the hydration is quicker in distilled water than in seawater, these results seem confirm that to obtain constant erosion during the paint lifetime, the hybrid paint should be able to absorb water continuously. However the results also show that erosion and degradation of hybrid polymers are not directly linked. The degradation of the copolymers goes on longtime after the end of erosion. 5.3.5. Surface properties To check the hybrid properties of binder due to the PDMS block, the surface hydrophobicity is monitored during immersion. The measurement of water contact angle of hybrid copolymers PCL 50s and 66s have been compared to a reference of poly(-caprolactone) homopolymer (Mn = 9000 g mol−1 ). Fig. 9 illustrates the results and shows a significant influence of PDMS block on surface properties. Both hybrid binders have a more hydrophobic (water contact angle >110◦ ) surface than the PCL reference (water contact angle <100◦ ). In a first stage (during the 100 first days of immersion), the hydrophobic properties of
both hybrid polymers decrease like for the reference polymer (PCL 9000). For the homopolymer PCL 9000, the decrease is observed while the water diffusion and polymer degradation is the most important. When the degradation decreases and the coating has reached the maximum of hydration, the contact angle of the reference remains constant at about 80◦ . The binders PCL 50 and PCL 66 have another behaviour. Their surface becomes more hydrophobic: their water contact angle increases to reach a value of 110◦ . The surface properties of PDMS become progressively predominant. So these results confirm the interest of this kind of binder that combines controlled erosion, biocides release and hydrophobicity during immersion. Furthermore, although hydrophobicity is a key parameter in the fouling release strategy, this parameter is not taken into account in the antifouling tests previously realized. Under hydrodynamic stress, the performances of hybrid paints are supposed to be higher. 6. Conclusion This work has shown the interest to develop hybrid binders. It is could be a new way to have antifouling paints more efficient and more environmental. The use of a copolymer with poly(caprolactone) and poly(dimethylsiloxane) as binder has done a paint that associates hydrophobicity and biodegradable properties. The corresponding paints are able to prevent fouling settlement and proliferation by (i) erosion, (ii) biocides release and (iii) high and constant hydrophobicity during one month of immersion. Among the parameters involved in the antifouling efficiency crystallinity, hydration, degradation, erosion and surface properties have been studied. It seems that crystallinity and hydration are key parameters to control the release of bioactive molecules in hybrid systems. But no link was observed between degradation and erosion. Some works is required on the formulation to have a better control on the biocides release and reduce hydration kinetics in order to increase the paint life. Furthermore, it should be interesting to test hybrid coatings in dynamic conditions to evaluate the PDMS contribution to antifouling activity. Acknowledgements This work was supported by the Region Bretagne (France) and PPG Coatings Europe. We sincerely thank Evonik Tego Chemie GmbH Company for the kind gift of the functional silicone monomer (Tegomer® H-Si 2311).
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References [1] C. Compère, M.N. Bellon-Fontaine, P. Bertrand, D. Costa, P. Marcus, C. Poleunis, C.M. Pradier, B. Rondot, M.G. Walls, Kinetics of conditioning layer formation on stainless steel immersed in seawater, Biofouling 17 (2001) 129–145. [2] M.P. Schultz, J.A. Bendick, E.R. Holm, W. Hertel, Economic impact of biofouling on a naval surface ship, Biofouling 27 (2011) 87–983. [3] D.M. Yebra, S. Kiil, K. Dam-Johansen, Antifouling technology – past, present and future steps towards efficient and environmentally friendly antifouling coatings, Prog. Org. Coat. 50 (2004) 75–104. [4] S. De Nancay, Nav. Ports Chantiers 39 (1989) 238. [5] M. Lejars, A. Margaillan, C. Bressy, Fouling release coatings: a nontoxic alternative to biocidal antifouling coatings, Chem. Rev. 112 (2012) 4347–4390. [6] K.A. Dafforn, J.A. Lewis, E.L. Jonhston, Antifouling strategies: history and regulation, ecological impacts and mitigation, Mar. Pollut. Bull. 62 (2011) 453–465. [7] IMO, International Convention on the Control of Harmful Antifouling Systems on Ships, International Maritime Organisation, London, 2001. [8] C.L. Alzieu, J. Sanjuan, J.P. Deltreil, M. Borel, Tin contamination in Arcachon bay: effect on oyster shell anomalies, Mar. Pollut. Bull. 17 (1986) 494–498. ¨ I. Linossier, K. Reh´ıel, Advances in Marine [9] C. Bressy, A. Margaillan, F. Fay, Antifouling Coatings and Technologies, Woodshead Publishing, Cambridge, UK, 2009, pp. 445. [10] C.M. Magin, S.P. Cooper, A.B. Brennan, Non-toxic antifouling strategies, Mater. Today 13 (4) (2010) 36–44. [11] M.P. Schultz, C.J. Kavanagh, G.W. Swain, Hydrodynamic forces on barnacles: implications on detachment from fouling-release surfaces, Biofouling 13 (1999) 323–335. [12] R.F. Brady, Fouling-release coatings for warships, Def. Sci. J. 55 (2005) 75–81. ¨ I. Linossier, V. Langlois, E. Renard, K. Vallée-Réhel, Degradation and [13] F. Fay, controlled release behavior of -caprolactone copolymers in biodegradable antifouling coatings, Biomacromolecules 7 (2006) 851–857. [14] J.M. Pelto, S. Virtanen, T. Munter, J. Larismaa, S. Jämsa, J. Nikkola, Encapsulation of 3-iodo-2-propynyl-N-butylcarbamate (IPBC) in polystyrenepolycaprolactone (PS/PCL) blends, J. Microencapsul. 31 (2014) 415–421. ˛ [15] M. Rutkowska, M. Jastrzebska, H. Janik, Biodegradation of polycaprolactone in sea water, React. Funct. Polym. 38 (1998) 27–30. [16] S.C. Woodward, P.S. Brewer, F. Moatamed, A. Schindler, C.G. Pitt, The intracellular degradation of poly-epsilon-caprolactone, J. Biomed. Mater. Res. 19 (1985) 437–444.
19
[17] R.A. Gross, B. Kalra, Biodegradable polymers for the environment, Science 297 (2002) 803–807. [18] K.J. Wynne, G.W. Swain, R.B. Fox, S. Bullock, J. Uilk, Two silicone nontoxic fouling release coatings: hydrosilation cured PDMS and CaCO3 filled, ethoxysiloxane cured RTV11, Biofouling 16 (2000) 277–288. [19] D. Wendt, G. Kowalke, J. Kim, I. Singer, Factors that influence elastomeric coating performance: the effect of coating thickness on basal plate morphology, growth and critical removal stress of the barnacle Balanus amphitrite, Biofouling 22 (2006) 1–9. [20] U. Buchholz, R. Mulhaupt, Branched polysiloxane block copolymers as epoxy toughening agents, ACS Polym. Prepr. 33 (1992) 205–206. ¨ K. Réhel, I. Linossier, Control of hydration and degradation [21] F. Azemar, F. Fay, properties of triblock copolymers polycaprolactone-b-polydimethylsiloxaneb-polycaprolactone, J. Appl. Polym. Sci. (2014) 2014. [22] M.F. Koenig, S.J. Huang, Biodegradable blends and composites of polycaprolactone and starch derivatives, Polymer 36 (1995) 1877–1882. [23] D. Haras, Biofilms and deteriorations of materials: from analysis of the phenomenon to prevention strategies, Mater. Tech. 93 (2006) s.27–s.41. [24] C. Bressy, J.F. Briand, C. Compère, K. Réhel, Efficacy testing of biocides and biocidal coatings, in: S. Dobretsov, J.C. Thomason, N.D. Wlliams (Eds.), Biofouling Methods, John Wiley & Sons, Oxford, 2014 (Chapter 12). [25] NF T 34-552, Peintures et vernis – Systèmes de peintures pour la protection des ouvrages en acier – Essai d’immersion au radeau en eau de mer vive, 1996. [26] D.E. Caldwaell, D.R. Korber, J.R. Lawrence, Confocal laser microscopy and digital image analysis in microbial ecology, Adv. Microb. Ecol. 12 (1992) 1–67. [27] E. Almeida, T.C. Diamantino, O. De Sousa, Marine paints: the particular case of antifouling paints, Prog. Org. Coat. 59 (2007) 2–20. [28] P. Castaing, Vieillisement des matériaux composites verre-polyester en milieu marin: délaminage d’origine osmotique (Thesis), 1992, pp. 34. [29] J.Z. Bei, J.M. Li, J.C. Le, S.G. Wang, Polycaprolactone-poly(ethylene-glycol) block copolymer. IV. Biodegradation behaviour in vitro and in vivo, Polym. Adv. Technol. 8 (1997) 693–696. [30] M.N. Kim, K.H. Kim, H.H. Jin, J.K. Park, J.S. Yoon, Biodegradability of ethyl and n-octyl branched poly(ethylene adipate) and poly(butylene succinate), Eur. Polym. J. 37 (2001) 1843–1847. ¨ I. Linossier, J.J. Peron, V. Langlois, K. Vallée-Réhel, Antifouling [31] F. Fay, activity of marine paints: study of erosion, Prog. Org. Coat. 60 (2007) 194–206.