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Correlation of mechanical properties with antifouling efficacy of coatings containing loaded microcapsules
Progress in Organic Coatings xxx (xxxx) xxxx
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Correlation of mechanical properties with antifouling efficacy of coatings containing loaded microcapsules P. Kavourasa, A.F. Trompetaa, S. Larrozeb, M. Maranhãob, T. Teixeirab, M. Beltrib, ⁎ E.P. Koumoulosa, C.A. Charitidisa, a
Research Unit of Advanced, Composite, Nanomaterials and Nanotechnology (R-NanoLab), School of Chemical Engineering, National Technical University of Athens, 9 HeroonPolytechniou Str., GR-15780 Athens, Greece b AquaBioTech Group, Central Complex Naggar Street, Targa Gap, Mosta MST 1761, Malta
A R T I C LE I N FO
A B S T R A C T
Keywords: Antifouling coatings Antifouling efficacy Nanomechanical properties Microcapsules Self-polishing properties
In this study, the antifouling efficacy of coatings containing loaded microcapsules made from gelatin and gum arabic, polyurethane, melamine-formaldehyde and a combination of the two first, is correlated with their mechanical properties. The nanoindentation technique is exploited for this reason, for the assessment of six antifouling coatings that have been exposed in the sea for nine months, in order to find a correlation between their antifouling functionality and their mechanical response. From the nanomechanical measurements, functional properties such as stiffness, wear resistance, elastic and plastic deformation, were extracted and connected to the antifouling performance of the coatings (leached layer, erosion rate, self-polishing properties, functionality maintenance). In parallel, surface characteristics were examined though optical microscopy, in order to detect defects and surface morphology that may affect the measurements. The antifouling efficacy was assessed with three different endpoints; biomass, chlorophyll a quantification and surface coverage. The above resulted to a classification of the novel antifouling coatings containing microcapsules, in terms of their antifouling performance and mechanical robustness. The results were also correlated with the release rate of the encapsulated biocide and compared with its addition in free state. It was concluded that the coating containing gelatin-gum arabic capsules had the best antifouling performance but not acceptable mechanical properties, whereas the coating containing polyurea capsules had both satisfying antifouling efficacy and improved mechanical properties compared to the control sample.
1. Introduction Biofouling is the unwanted accumulation and growth of a variety of deposits, especially living organisms, on a submerged surface [1,2]. The process of biofouling formation consists of the successive colonization of a surface by a large variety of organisms and it is a highly dynamic process, varying the degree and type of fouling depending on geographical, physical and chemical factors, as well as species diversity and environmental factors [3–5]. Causing serious problems for a wide range of marine industries and navies around the world [6], different anti-fouling treatments have been developed to prevent the build-up of biofouling. Currently, chemical treatments or biocides are the most widely applied antifouling agents as a means to prevent and control the fouling process [7]. Thus, anti-fouling coatings have been developed to incorporate active elements such as metallic antifoulants and organic booster biocides
⁎
[7]. In that instance, self-polishing antifouling coatings work by continuously releasing biocides from the paint surface at a rate sufficient to produce a toxic concentration on the surface layer. Antifouling coatings for the shipping sector are designed to last up to five years to avoid frequent dry docking. The aim of antifouling paint research should be to develop coatings which are effective for longer periods without increasing marine pollution [6]. The durability of the coating depends upon its resistance to mechanical damage, the erosive effects of water movement and the softening or dissolution of the components of the paint [8]. Since selfpolishing coatings must disintegrate slowly to permit the leaching of biocides, a compromise must be made between toxicity and durability. In order to achieve prolonged antifouling protection for a long service life, a careful control of the erosion rate is needed, with the ultimate aim to avoid any consumption of the coating. This is an ideal situation, which cannot be achieved in real life, since a gradual wasting of the
Corresponding author. E-mail address: [email protected] (C.A. Charitidis).
https://doi.org/10.1016/j.porgcoat.2019.105249 Received 18 December 2018; Received in revised form 10 June 2019; Accepted 20 July 2019 0300-9440/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: P. Kavouras, et al., Progress in Organic Coatings, https://doi.org/10.1016/j.porgcoat.2019.105249
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material volume [22]. Nanoindentation also provides worthy information on the local mechanical properties at sub-microscale [19]. Nonuniform distribution of fillers can be readily monitored by such techniques, since modulus and hardness values may vary over a broad range of indentation depths [18,23]. In this study, the nanoindentation technique is exploited for the assessment of antifouling coatings that have been exposed in the sea for nine months, in order to find a correlation between their antifouling functionality and their mechanical response. To the best of our knowledge, there is no such correlation in existing literature, yet.
paint is also needed for the release of the biocides. However, coatings with improved mechanical strength can last longer during their operation. The rate of dissolution and wear is a way to determine the life of antifouling paints. Creating a paint that is too soft (i.e. having poor mechanical integrity) when dry, will result in polishing away rapidly, revealing the inert undercoating and making it accessible to fouling. In combination with the resin system chosen, different pigments, metal oxides in common, are incorporated into the film at the product design level to give the right balance of mechanical strength and solubility. This ensures that the required time in service life is achieved [9]. Booster biocides, such as SeaNine 211,complement the performance of the conventional copper-based paints [10,11]. Booster biocides have a different mechanism of action compared to the copper-based ones. They are mostly organic molecules, which easily dissolve in the coating formulation. After coating curing they are homogenously distributed or “dissolved” in the polymer matrix of the coating. As a result, they can be released comparatively fast resulting in premature leakage. SeaNine 211 consists of a 30% solution of 4,5-dichloro-2-n-octyl-4-isothiazolin3-one (DCOIT) in mixed xylenes and belongs to the new generation of environmentally acceptable marine antifouling agents. According to the technical specifications of the product, SeaNine 211 remains stable for at least two years at 22–25 °C and 6 months at 40 °C, enabling its encapsulation in polymeric materials. All physiochemical properties of DCOIT are included in [12]. It has a broad spectrum activity against bacterial slime, algae, barnacles, tubeworms, hydroids, bryozoan, tunicates and diatoms while having a minimal bioaccumulation of toxicologically significant compounds [10,12]. Under normal use patterns, concentrations of 3–10% are sufficient. The encapsulation of booster biocides in microcapsules is an innovative technique that enables the controlled release of the biocide [2,13], as the biocide in the microcapsules acquires a pigment-like state. The biocide can be slowly released from the microcapsules’ surface and dissolved in the coating, after which it can diffuse through the coating to its surface. Encapsulation of booster biocides can also lower the environmental impact of the coating without compromising its antifouling properties. Laboratory tests on mussels indicated that encapsulation of booster biocides could potentially improve the antifouling efficacy of the biocides while decreasing their toxicity effects [2,14]. The reinforcement of paint matrix with nanoparticles can also improve the mechanical properties in the coating [15] such as tensile strength, flexibility and impact resistance. The common technique used for the measurement of mechanical strength is the impact test, performed according to ASTM D 2794 [16]. However, traditional methods need bulky standard samples to evaluate the mechanical behaviour of materials and as a consequence are not applicable in this case [17]. Moreover, in contrast to homogeneous materials, nanocomposite materials are much more complex in their microstructure and mechanical performance, as well as being time-dependent in their load response [18]. Therefore, advanced mechanical testing techniques, such as nanoindentation have become a commonplace tool for the measurement of mechanical properties at sub-micron scale [18,19]. The method developed by Oliver and Pharr allows determining the hardness and the elastic modulus from the nanoindentation load–displacement data [20]. The indentation response in the form of force-penetration (P-h) curves is characterized by two elastic constants, indentation modulus (through stiffness) and indentation hardness. Direct application of these equations to heterogeneous materials poses several difficulties, as the underlying analysis relies on the self-similarity of the indentation test which holds only for homogeneous materials. The interaction of phases in heterogeneous materials is unavoidable but depending on the length scale it can be more or less important. Properties extracted from indentation data of a heterogeneous solid can be treated as averaged quantities dependent on the depth [21]. Therefore, the choice of an indentation depth directly determines the length scale of the tested
2. Materials and methods 2.1. Microencapsulation of the booster biocide “SeaNine 211” Polysaccharide-based encapsulation technology was used for the preparation of the modified antifouling coatings, which were provided by Fraunhofer Institute for Applied Polymer Research IAP. Specifically, complex coacervation has been chosen as the suitable encapsulation technique [24]. The technology is based on the complex formation between a protein and oppositely charged polysaccharide and is already exploited in a few application fields on the industrial scale [24,25]. After the reaction of complex formation is completed, the system exists in the form of an aqueous suspension of hydrogel micro particles enclosing liquid core material (immiscible with water) [13]. The mechanical stability and the barrier properties of the capsule shells are comparatively poor at this stage. After drying, the shell of the particles becomes hard (but fragile), while the barrier properties of the shell are considered to be very good. Poor mechanical stability of the shell can be considered to be the main disadvantage of the technology [13]. The materials provided by Fraunhofer IAP were designed to have aninner supporting wall with good mechanical stability and simultaneously good permeability towards SeaNine, and an outer polysaccharide-based layer with suitable barrier properties. In total, four different encapsulations were tested in: gelatin and gum arabic (GGA) [26,27], polyurethane (PU) [28], melamine-formaldehyde (MF) [29] and a combination of PU and GGA (double shells) microcapsules [30]. The detailed synthesis description of the microcapsules in not in the scope of this paper, thus the relevant literature references have been provided for each of the microcapsules used in this study.
2.2. Coatings preparation SeaNine booster biocide was encapsulated in the four aforementioned types of capsules, which were mixed into tested standard selfpolishing coating formulation (Test product), provided by JOTUN S.A., containing no biocides. These new capsule-based paint formulations were applied on PVC plates using a paint applicator with a 400 μm gap size. The content of microcapsules in the Test product was adjusted to correspond to 2 wt.% of the active in the final coating. According to this, initially a formulation containing the active in a dissolved state was prepared (FSI). Then, GGA/SeaNine capsules were incorporated in the Test product at 6 wt.% (FSII), PU/SeaNine capsules at 6 wt.% (FSIII), PU/GGA/SeaNine capsules at 8 wt.% (FSIV) and finally MF capsules at 6 wt.% (FSV). Coating formulations containing no biocide at all were also prepared for comparison (Control sample).
2.3. Physical conditions The physical performance of the panels was visually assessed before immersion following international guidelines [31]. The coatings were visually inspected for cracking, peeling, blistering, wearing and pores or holes. Coatings displaying a smooth surface were judged as optimal. 2
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The coated PVC panels were immersed in the central-south Mediterranean waters in an industrial harbour of Malta (35°54′31.5″N; 14°23′34.0″E) from June 2015 until March 2016. The panels were fastened on a fibreglass board fixed to a marina quay and submerged at 1.5–3 m depth. The seawater temperature, pH, salinity, conductivity and dissolved oxygen concentration were monthly monitored with a portable meter Hach, HQ30d.
performed using the statistical package R [36]. Before analysis, the different sets of data were checked for assumption of normality and equality of variance using the Kolmogorov-Smirnov one-sample test and the Levene test, respectively. Wherever both assumptions were met, data were analysed by one-way ANOVA followed by Tukey HSD post hoc test. Data was analysed by the non-parametric Kruskal-Wallis test followed by the Nemenyi post-hoc test whenever either of the assumptions was not met. Differences were considered significant at p < 0.05.
2.5. Antifouling efficacy
2.6. Surface characterization under optical microscopy
2.5.1. Antifouling efficacy parameter At the end of immersion time, the panels were retrieved and gently rinsed with seawater from the site to remove excess silt [32]. A semiquantitative antifouling efficacy analysis of the coatings was carried out [32,33]. Aquadrant with 8 central divisions and a delimitation of 1 cm from the panel’s edge was placed over each panel to help in the visual assessment and recording of the estimated percentage of colonized surface. The antifouling performance of the tested paints was calculated using the Antifouling Efficacy parameter – “N parameter” as described by Bressy et al. [33]. The formula combines the composition, type of fouling and percentage of area covered:
The surface of each coating was analyzed through light microscopy (reflectance mode, dark field and polarised filters, magnification x100), using an AxioImager.A2m microscope with Zeiss lens. Three different points of the surface of each sample were examined. Special attention was given to defects and points of interest.
2.4. Immersion site
N=
∑ (IF
2.7. Nanomechanical properties measurements A Hysitron Tribolab® Instrument, equipped with a standard threesided pyramidal Berkovich tip, was used for the nanomechanical tests. Displacement resolution (0.04 nm) and load resolution (1 nN) at the vertical direction were high enough to allow detailed characterization of nanomechanical properties. Nanomechanical testing was applied in a clean area environment with 45% humidity and 23 °C ambient temperature. A standard fused silica sample was used to calibrate the area function of the Berkovich indenter tip before the application of measurements. All indentations were obtained with a single load-unload scheme, under Displacement Control (DC) mode; at DC mode the indenter tip is inserted into the sample surface in a predefined depth with simultaneous recording of the applied load. In the case of the present work all indentations were made at maximum depth of 200 nm. This depth was high enough to achieve an elastoplastic contact, without exceeding the 1/10th of the upper coating layer; as a result, plastic deformation was limited at the upper coating layer, i.e. from which nanomechanical data were extracted. Each indentation was made with a load and unload ramp of 40 and a dwell time at the maximum load of 3 s.
× SF )
Where IF is the intensity factor related to the percentage of cover for each type of organism and SF is the severity factor for each type of organism (SFslime = 1 and SFEncrusting animals = 6). 2.5.2. Fouling coverage A photo record of the test panels was also made with a camera Canon IXUS 155. The camera settings and focal distance were kept constant through time. Focal distance consistency was achieved using a support to hold the camera. The image processing software Image J [34] was used to estimate the percentage of area covered by the fouling community. Digital segmentation [1] of the images - isolation of the background colour from the components of interest- was used. 2.5.3. Wet weight The panels were placed in a plastic bag with seawater from the site and transported in a cooled box. In the laboratory, panels were placed vertically to drain the excess water for 3–6 min and weighted. Biomass was estimated as the difference between the wet weight of the panel covered with fouling and the initial weight of the panel. The results are expressed as the weight of the area covered by fouling community (mg/cm2) [3].
3. Results and discussion 3.1. Field immersion trials Throughout the experiment the temperature fluctuated between 15.7–27.7 °C (see Table 1) pH and salinity remained fairly constant. The physical condition of the control, FSI, FSIII, FSIV and FSV coatings were visually smooth before and after one month of immersion (see Table 2). FSII displayed a grainy texture.
2.5.4. Chlorophyll a content Fouling was scraped from panels on three squares of 1 cm2 with a spatula and transferred into falcon tubes (one square with heavy fouling, one with mild and one with little fouling). Samples were extracted in 90% acetone HPLC grade at 8 °C for 12–16 h in the dark, followed by centrifugation for 10 min. Chl–a was determined using spectrophotometry following the method described by Aminot and Rey [35], using a Thermoscientific spectrophotometer Genesis 10. The formula was adapted as follows:
3.2. Semi-quantitative antifouling efficacy After 9 months of exposure, the control coatings were heavily fouled, mainly by calcareous tubeworms, encrusted and branchy Table 1 Environmental parameters of the Mediterranean seawater throughout the field trial.
Chl–a = (11.85 × (E 664 − E 750) − 1.54 × (E 647 − E 750) Ve − 0.08 × (E 630 − E 750)) × ×L S Where: L = Cuvette light-path in cm, Ve = Extraction volume in mL, S = Surface area scraped in cm2. Concentrations are in unit mg/cm2. 2.5.5. Statistical analysis Statistical techniques to determine significant results were 3
Parameter
Average ( ± sd)
Min
Max
Temperature (°C) Salinity (‰) Conductivity (mS/cm) pH O2 (mg/L) Visibility (m)
21.63 ( ± 5.11) 39.29 ( ± 2.29) 55.58 ( ± 7.48) 8.10 ( ± 0.07) 7.95 ( ± 1.14) 2.83 ( ± 0.83)
15.70 35.40 44.70 8.01 6.48 2.00
27.70 41.70 64.50 8.22 9.50 4.00
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Table 2 Antifouling performance of coated panels immersed for 9 months in the Mediterranean Sea. N = 3. Significant differences between treatment and control groups are shown with *. Significant differences between coatings with microcapsules and FSI are indicated with +. Sample
Biomass ± sd (mg/ cm2)
Chlorophyll a ± sd (mg/ cm2)
Antifouling efficacy N ± sd
Fouling coverage ± sd (%)
Physical condition before immersion
Test Product without biocides (Control)
114.97 ( ± 4.93) 62.76 ( ± 24.09) 35.70 ( ± 6.98)* 54.81 ( ± 13.30) 76.09 ( ± 19.3) 80.80 ( ± 48.90)
7.31 ( ± 0.78) 3.62 ( ± 0.58)*** 3.36 ( ± 0.59)*** 3.97 ( ± 0.32) 5.56 ( ± 0.93)+ 5.63 ( ± 0.71)+
39.33 ( ± 10.11) 34.00 ( ± 6.25) 6.33 ( ± 2.31)*** 29.33 ( ± 4.62) 26.00 ( ± 6.24) 23.67 ( ± 7.50)
98.03 ( ± 0.79) 93.34 ( ± 4.77) 88.97 ( ± 3.95)* 96.38 ( ± 1.31) 91.57 ( ± 5.03) 94.48 ( ± 3.43)
smooth
4 wt% SeaNine in Test Product (FSI) 6 wt% GGA_SeaNine capsules in Test product (FSII) 6 wt% PU_SeaNine capsules in Test product (FSIII) 8 wt% PU/GGA_SeaNine capsules in Test product (FSIV) 6 wt% MF_SeaNine capsules in Test product (FSV)
Significant differences between treatment and control groups are shown with *. The
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smooth grainy smooth smooth smooth
correspond to Tukey HSD post-hoc test with p = 5.9 × 104.
Fig. 1. Monthly progression of colonization by fouling organisms on coated panels in the Mediterranean Sea. Photographs of one replicate panel per treatment groups are displayed from first until last day of immersion.
FSIV (Tukey HSD post-hoc test, p < 0.032). The absence of encrusting organisms on FSII greatly decreased the N parameter since the severity factor of slime is 1.
bryozoan, barnacles, bivalves and green algae as seen in Fig. 1. Visual observation indicated the control coatings were colonised by a higher quantity of fouling organisms but also displayed a superior specie richness compared with the biocidal coatings (FSI-FSV). The percentage of hard fouling organisms (i.e. barnacles, bivalves, calcareous tubeworms, encrusting bryozoan) was greater on the control coatings than on the biocidal coatings as revealed by the semi-quantitative antifouling efficacy analysis (see Fig. 2). The control coating was mainly colonized by hard fouling organisms (> 50%) and displayed little bare space (Figs. 1 and 2). FSI, FSIII and FSIV were colonized by slime and hard fouling. FSV was colonized by slime and soft fouling while FSII displayed mainly slime. The N parameter, based on semi-quantitative observations, was the highest on the control coatings and the lowest on FSII (Table 2). No significant differences were identified between the N parameter of the control and FSI, FSIII, FSIV or FSV coatings (Tukey HSD post-hoc test, p > 0.1). On the other hand, FSII significantly decreased the N parameter compared to the control coatings (Tukey HSD post-hoc test, p = 5.9 × 10−4). FSII coatings also showed a lower N parameter compared to FSI, FSIII or
3.3. Quantitative antifouling efficacy The highest fouling coverage (98.03 ± 0.79) was observed on the control coatings while FSII displayed the lowest coverage (88.97 ± 3.95, Table 2). No significant differences were found between the fouling coverage percentage of the control and FSI, FSIII, FSIV or FSV coatings (Nemenyi post-hoc test, p > 0.19). On the other hand, FSII displayed a lower fouling coverage compared to control coatings (Nemenyi post-hoc test, p = 0.043). FSII coatings accumulated significantly less fouling biomass compared with the control coatings (Tukey HSD post-hoc test, p = 0.018). No significant differences were observed between the fouling biomass accumulated on FSI, FSIII, FSIV or FSV and the control coatings (Tukey HSD post-hoc test, p > 0.08). Similarly, no significant differences were found between FSII, FSIII, FSIV or FSV and FSI (Tukey HSD post-hoc test, p > 0.7). 4
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Fig. 2. Average percentage fouling coverage on coatings by category after 9 months of immersion in the Mediterranean Sea. N = 3.
curves (Fig. 3). The assessment of the nanomechanical properties is based on comparisons with the control sample, as the reference one. The control sample showed relatively good mechanical properties, presenting also the smoothest surface, which was reflected on the absence of outliers, while all load-displacement curves (except one) did not contain any discontinuities (see Fig. 3). FSI sample showed increased Er; however, the measured hardness was lower. The measurements produced also relatively more outliers that are possibly connected with the addition of SeaNine in pure state, which may cause “softening” points on the samples surface. FSII sample presented the lowest mechanical properties of all coatings. FSIII sample showed higher Er and H values, with the surface being rougher than the control, and a number of outliers between the control and FSI. FSV, as FSII, showed also lower values of Er and H than the control, and in parallel, both depicted high surface roughness and the highest number of outliers. At this point, it should be mentioned that the number of outliers indicates the inhomogeneity of the coatings surface, since the measurements on specific points was impossible. This can be attributed to the holes, pores and craters that were observed on the surface of the coatings.
FSI, FSII and FSIII displayed significantly less chlorophyll a, compared to control coatings (Tukey HSD post-hoc test, p < 3 × 10−4). Significantly more chlorophyll a was found on FSIV and FSV compared with FSI or FSII (Tukey HSD post-hoc test, p < 0.04). Overall, FSII displayed the most optimal antifouling efficacy compared to control coatings and also to FSI. FSIII displayed good antifouling efficacy while FSIV and FSV displayed limited antifouling efficacy. 3.4. Assessment of exposed surface Prior the nanomechanical testing, all samples were examined under optical microscopy in order to detect any possible defect that may affect the results. After the antifouling assessment, the specimens were rinsed with water and carefully cleaned, in order to remove any slime and residues of the fouling organisms. Representative images of the coatings surface are presented in Table 3. The control sample presents a smooth and homogenous surface with fading on several places. On the other hand, on FSI sample residues of microorganisms were detected (in black colour), as well as severe fading. Sample FSII presented a surface full of holes and craters of about 200 μm. Agglomerates also of the GGA capsules were detected in a yellowish jelly state. FSIII sample’s surface was full of PU particles, forming a porous and loose surface. Holes around 200 μm were also observed. On this sample, residues of fouling organisms (possible algae) were also detected in a linear morphology. FSIV sample presented the most heterogenous surface, full of particles similar to those observed on sample FSIII. Huge craters with a diameter of more than 760 μm were also detected. Finally, sample FSV had the most homogenous surface among the modified coatings with only some pores of ˜200 μm on some places. The morphology of the surface was quite close to the control’s sample. From the assessment of the exposed surface of all specimens it may be remarked that PU capsules form agglomerates. Also, all modified coatings presented increased porosity in respect to the control sample and FSI.
3.6. Overall comparative assessment The comparative results are presented in Fig. 6 and Table 5 below, which summarises the antifouling and mechanical properties, trying to classify the performance of all coatings, in terms of antifouling efficacy and mechanical integrity. Control sample without any biocide presented the worst antifouling performance, as expected, revealing on the other hand good mechanical properties, which were considered as the basis of the assessment. For the case of FSI, due to the presence of SeaNine in pure form, a 50% reduction of the detected chlorophyll was observed in comparison with the control sample. This sample proved to have also increased stiffness (8.51 μN/nm compared to 6.29 μN/nm of the control), which affects the leaching layer of the coating. The stiffer the coating is, the harder to self-polish. This means that the hydrolysis rate will be lower and as a result, the overall antifouling performance is not as high as expected. FSII presented the best antifouling performance for all endpoints. However, this coating may fail in the long term, since the release/ polishing rate is intense, considering the downgraded mechanical properties. Specifically, this sample presented the highest plastic
3.5. Nanomechanical properties results Samples that have been exposed in the field were examined with the nanoindentation technique. Table 4 includes the hardness values (H) and reduced modulus values (Er) extracted from the load-displacement 5
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Table 3 Surface assessment of tested samples. Dark field
Sample
Polarised field
Test Product without biocides (Control)
4 wt% SeaNine in Test Product (FSI)
6 wt% GGA_SeaNine capsules in Test product (FSII)
6 wt% PU_SeaNine capsules in Test product (FSIII)
8 wt% PU/GGA_SeaNine capsules in Test product (FSIV)
6 wt% MF_SeaNine capsules in Test product (FSV)
deformation which is calculated from the measured hardness, according to [37]:
Plastic Deformation = 1 −
However, the increased plastic deformation which is correlated with the self-polishing performance, indicates that the coating will be rabidly eroded, losing its effectiveness. On the other hand, FSIII exhibited good antifouling performance, with high mechanical integrity and
h max − hc h max
Table 4 Assessment of the data obtained from the nanoindentation testing. Sample
Er ± sd (GPa)
H ± sd (MPa)
H/E ± sd
Outliers
Test Product without biocides (Control) 4 wt% SeaNine in Test Product (FSI) 6 wt% GGA_SeaNine capsules in Test product (FSII) 6 wt% PU_SeaNine capsules in Test product (FSIII) 8 wt% PU/GGA_SeaNine capsules in Test product (FSIV) 6 wt% MF_SeaNine capsules in Test product (FSV)
7.12 ± 2.48 12.61 ± 9.03 0.47 ± 0.28 9.65 ± 0.84 2.05 ± 0.76 2.65 ± 0.28
835.4 ± 271.9 600.7 ± 387 21.9 ± 15 965 ± 91.7 235.3 ± 210.3 175.2 ± 57.4
0.12 0.06 0.04 0.10 0.12 0.07
0 3 3 2 1 3
For comparison reasons, the extracted values are depicted in bar charts form, in Figs. 4 and 5, respectively. 6
± ± ± ± ± ±
0.02 0.03 0.01 0.01 0.06 0.01
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Fig. 3. Characteristic load-displacement curves.
4. Conclusions
robustness, observed through load-displacement curves homogeneity (from Fig. 3 it can be noticed that the curves are not spread, but close to each other). Moreover, the increased elastic deformation, calculated also from the measured hardness, according to [37]:
Elastic Deformation =
This study aimed at a correlation between antifouling performance and nanomechanical properties of antifouling coatings containing loaded microcapsules. The results of the antifouling assessment have shown that the performance of the microcapsules made from different material combinations, can vary from the one coating system to the other. Since all coatings contain the same amount of biocide (active ingredient), it can be concluded that encapsulation of the biocide does allow the control of the release rate and as a result the antifouling efficacy. This was connected with the measured mechanical properties of the coatings, specifically the elastic and plastic deformation, as well as the stiffness and resistance to wear -connected to the release/erosion rate-. The slowest release was observed from melamine-formaldehyde microcapsules, and polyurethane/gelatine-gumarabic double walled
h max − hc h max
indicates a good functionality maintenance. This sample presented also the highest stiffness, with a low standard deviation. FSIV exhibited reduced antifouling performance simultaneously with a high wear resistance, proved by the high H/E ratio [38]. Finally, FSV presented the worst antifouling and mechanical performance compared to the rest coatings, indicating that the addition of MF capsules deteriorates the properties of the paint. 7
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Fig. 4. Coatings hardness.
Fig. 5. Coatings Young’s modulus.
Fig. 6. Comparative graph of mechanical and antifouling performance. 8
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Poor Performance).
Plastic Deformation
microcapsules which showed similar antifouling performances. Polyurethane microcapsules did not impact the release rate and the mechanical properties were similar with the coating containing the biocide in a free state. Gelatine-gum arabic microcapsules showed the fastest release, resulting in the highest antifouling performance, without expecting however its long-term efficacy. From the above it may be concluded that a compromise between mechanical and antifouling performance is necessary when considering the overall functionality of the coating. Additionally, the surface characteristics play also a crucial role, since the porosity and roughness of the surface affect the release of the biocide and the attachment of the fouling organisms, respectively. Assessing mechanical as well as antifouling properties of coatings helps predicting their long-term efficacy.
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[1] S. Dobretsov, J.C. Thomason, D.N. Williams, Biofouling Methods, John Wiley & Sons, Ltd., Oxford, UK, 2014, https://doi.org/10.1002/9781118336144 ISBN 9781118336144. [2] E. Gutner-Hoch, R. Martins, T. Oliveira, F. Maia, A. Soares, S. Loureiro, C. Piller, I. Preiss, M. Weis, S. Larroze, T. Teixeira, J. Tedim, Y. Benayahu, Antimacrofouling efficacy of innovative inorganic nanomaterials loaded with booster biocides, J. Mar. Sci. Eng. 6 (1) (2018) 6, https://doi.org/10.3390/jmse6010006. [3] I. Menchaca, I. Zorita, N. Rodríguez-Espeleta, E. Erauskin, C. Erauskin, P. Liria, M. Santesteban, I. Mendibil, I. Urtizberea, Guide for the evaluation of biofouling formation in the marine environment, Rev. Investig. Mar. 21 (2014) 90–99. [4] R.A. Braithwaite, L.A. McEvoy, Marine biofouling on fish farms and its remediation, Adv. Mar. Biol. 47 (2004) 215–252, https://doi.org/10.1016/S0065-2881(04) 47003-5. [5] C. Hellio, D. Yebra, Advances in Marine Antifouling Coatings and Technologies, (2009), https://doi.org/10.1533/9781845696313 ISBN 9781845693862. [6] 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, https://doi.org/10.1016/j.porgcoat.2003.06. 001. [7] C.M. Kirschner, A.B. Brennan, Bio-inspired antifouling strategies, Annu. Rev. Mater. Res. 42 (2012) 211–229, https://doi.org/10.1146/annurev-matsci-070511155012. [8] Woods Hole Oceanographic Institute Characteristics of Antifouling Coatings, Marine Fouling and Its Prevention, George Banta Publishing Co., 1952, pp. 313–322. [9] CEPE Function of Zinc Oxide in Antifouling Paints, (2010). [10] K.V. Thomas, S. Brooks, The environmental fate and effects of antifouling paint biocides, Biofouling 26 (2010) 73–88, https://doi.org/10.1080/ 08927010903216564. [11] L.D. Chambers, K.R. Stokes, F.C. Walsh, R.J.K. Wood, Modern approaches to marine antifouling coatings, Surf. Coat. Technol. 201 (2006) 3642–3652, https://doi.org/ 10.1016/j.surfcoat.2006.08.129. [12] L. Chen, J.C.W. Lam, SeaNine 211 as antifouling biocide: a coastal pollutant of emerging concern, J. Environ. Sci. 61 (2017) 68–79, https://doi.org/10.1016/j.jes. 2017.03.040. [13] R.L. Hart, D.R. Virgallito, D.E. Work, Microencapsulation of biocides and antifouling agents, Microtek Laboratories Inc. (Dayton, OH, US), USA, 2011. [14] F. Avelelas, R. Martins, T. Oliveira, F. Maia, E. Malheiro, A.M.V.M. Soares, S. Loureiro, J. Tedim, Efficacy and ecotoxicity of novel anti-fouling nanomaterials in target and non-target marine species, Mar. Biotechnol. 19 (2017), https://doi. org/10.1007/s10126-017-9740-1. [15] J. Dustebek, C. Kandemir-Cavas, S.F. Nitodas, L. Cavas, Effects of carbon nanotubes on the mechanical strength of self-polishing antifouling paints, Prog. Org. Coat. 98 (2016) 18–27, https://doi.org/10.1016/J.PORGCOAT.2016.04.020. [16] ASTM International, Standard Test Method for Resistance of Organic Coatings to the Effects of Rapid Deformation (Impact), ASTM International, 2015, pp. 12–14, https://doi.org/10.1520/D2794-93R10 06.01. [17] I.A. Kartsonakis, E.P. Koumoulos, A.C. Balaskas, G.S. Pappas, C.A. Charitidis, G.C. Kordas, Hybrid organic–inorganic multilayer coatings including nanocontainers for corrosion protection of metal alloys, Corros. Sci. 57 (2012) 56–66, https:// doi.org/10.1016/J.CORSCI.2011.12.034. [18] E.P. Koumoulos, D.A. Dragatogiannis, I.A. Kartsonakis, E. Karaxi, T. Kehagias, C.A. Charitidis, Nanomechanical performance of protective coatings reinforced
**
**
**** ****
***
*
** ****
****
*
**** * *
Test Product without biocides (Control) 4 wt% SeaNine in Test Product (FSI) 6 wt% GGA_SeaNine capsules in Test product (FSII) 6 wt% PU_SeaNine capsules in Test product (FSIII) 8 wt% PU/GGA_SeaNine capsules in Test product (FSIV) 6 wt% MF_SeaNine capsules in Test product (FSV)
***
Anti-macrofouling efficacy
*
Stiffness (μN/nm) Resistance to wear (H/E)
This work was supported by the EU FP7 Project “Low-toxic costefficient environment-friendly antifouling materials” (BYEFOULING) under Grant Agreement no. 612717. The authors would like to thank Jotun S.A. for providing the benchmarked coatings, as well as Dr. Alexandra Latnikova from Fraunhofer Institute for Applied Polymer Research IAP, for the modified coatings with the encapsulated biocides. References
Biofilm inhibition efficacy
Overall Antifouling efficacy
Acknowledgments
Coating
Table 5 Ranking of mechanical properties with antifouling efficacy of coatings exposed in the Mediterranean Sea for 9 months. (**** Optimal Performance,
***
Good Performance,
**
Limited Performance,
*
Elastic Deformation
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Contents lists available at ScienceDirect
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Correlation of mechanical properties with antifouling efficacy of coatings containing loaded microcapsules P. Kavourasa, A.F. Trompetaa, S. Larrozeb, M. Maranhãob, T. Teixeirab, M. Beltrib, ⁎ E.P. Koumoulosa, C.A. Charitidisa, a
Research Unit of Advanced, Composite, Nanomaterials and Nanotechnology (R-NanoLab), School of Chemical Engineering, National Technical University of Athens, 9 HeroonPolytechniou Str., GR-15780 Athens, Greece b AquaBioTech Group, Central Complex Naggar Street, Targa Gap, Mosta MST 1761, Malta
A R T I C LE I N FO
A B S T R A C T
Keywords: Antifouling coatings Antifouling efficacy Nanomechanical properties Microcapsules Self-polishing properties
In this study, the antifouling efficacy of coatings containing loaded microcapsules made from gelatin and gum arabic, polyurethane, melamine-formaldehyde and a combination of the two first, is correlated with their mechanical properties. The nanoindentation technique is exploited for this reason, for the assessment of six antifouling coatings that have been exposed in the sea for nine months, in order to find a correlation between their antifouling functionality and their mechanical response. From the nanomechanical measurements, functional properties such as stiffness, wear resistance, elastic and plastic deformation, were extracted and connected to the antifouling performance of the coatings (leached layer, erosion rate, self-polishing properties, functionality maintenance). In parallel, surface characteristics were examined though optical microscopy, in order to detect defects and surface morphology that may affect the measurements. The antifouling efficacy was assessed with three different endpoints; biomass, chlorophyll a quantification and surface coverage. The above resulted to a classification of the novel antifouling coatings containing microcapsules, in terms of their antifouling performance and mechanical robustness. The results were also correlated with the release rate of the encapsulated biocide and compared with its addition in free state. It was concluded that the coating containing gelatin-gum arabic capsules had the best antifouling performance but not acceptable mechanical properties, whereas the coating containing polyurea capsules had both satisfying antifouling efficacy and improved mechanical properties compared to the control sample.
1. Introduction Biofouling is the unwanted accumulation and growth of a variety of deposits, especially living organisms, on a submerged surface [1,2]. The process of biofouling formation consists of the successive colonization of a surface by a large variety of organisms and it is a highly dynamic process, varying the degree and type of fouling depending on geographical, physical and chemical factors, as well as species diversity and environmental factors [3–5]. Causing serious problems for a wide range of marine industries and navies around the world [6], different anti-fouling treatments have been developed to prevent the build-up of biofouling. Currently, chemical treatments or biocides are the most widely applied antifouling agents as a means to prevent and control the fouling process [7]. Thus, anti-fouling coatings have been developed to incorporate active elements such as metallic antifoulants and organic booster biocides
⁎
[7]. In that instance, self-polishing antifouling coatings work by continuously releasing biocides from the paint surface at a rate sufficient to produce a toxic concentration on the surface layer. Antifouling coatings for the shipping sector are designed to last up to five years to avoid frequent dry docking. The aim of antifouling paint research should be to develop coatings which are effective for longer periods without increasing marine pollution [6]. The durability of the coating depends upon its resistance to mechanical damage, the erosive effects of water movement and the softening or dissolution of the components of the paint [8]. Since selfpolishing coatings must disintegrate slowly to permit the leaching of biocides, a compromise must be made between toxicity and durability. In order to achieve prolonged antifouling protection for a long service life, a careful control of the erosion rate is needed, with the ultimate aim to avoid any consumption of the coating. This is an ideal situation, which cannot be achieved in real life, since a gradual wasting of the
Corresponding author. E-mail address: [email protected] (C.A. Charitidis).
https://doi.org/10.1016/j.porgcoat.2019.105249 Received 18 December 2018; Received in revised form 10 June 2019; Accepted 20 July 2019 0300-9440/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: P. Kavouras, et al., Progress in Organic Coatings, https://doi.org/10.1016/j.porgcoat.2019.105249
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material volume [22]. Nanoindentation also provides worthy information on the local mechanical properties at sub-microscale [19]. Nonuniform distribution of fillers can be readily monitored by such techniques, since modulus and hardness values may vary over a broad range of indentation depths [18,23]. In this study, the nanoindentation technique is exploited for the assessment of antifouling coatings that have been exposed in the sea for nine months, in order to find a correlation between their antifouling functionality and their mechanical response. To the best of our knowledge, there is no such correlation in existing literature, yet.
paint is also needed for the release of the biocides. However, coatings with improved mechanical strength can last longer during their operation. The rate of dissolution and wear is a way to determine the life of antifouling paints. Creating a paint that is too soft (i.e. having poor mechanical integrity) when dry, will result in polishing away rapidly, revealing the inert undercoating and making it accessible to fouling. In combination with the resin system chosen, different pigments, metal oxides in common, are incorporated into the film at the product design level to give the right balance of mechanical strength and solubility. This ensures that the required time in service life is achieved [9]. Booster biocides, such as SeaNine 211,complement the performance of the conventional copper-based paints [10,11]. Booster biocides have a different mechanism of action compared to the copper-based ones. They are mostly organic molecules, which easily dissolve in the coating formulation. After coating curing they are homogenously distributed or “dissolved” in the polymer matrix of the coating. As a result, they can be released comparatively fast resulting in premature leakage. SeaNine 211 consists of a 30% solution of 4,5-dichloro-2-n-octyl-4-isothiazolin3-one (DCOIT) in mixed xylenes and belongs to the new generation of environmentally acceptable marine antifouling agents. According to the technical specifications of the product, SeaNine 211 remains stable for at least two years at 22–25 °C and 6 months at 40 °C, enabling its encapsulation in polymeric materials. All physiochemical properties of DCOIT are included in [12]. It has a broad spectrum activity against bacterial slime, algae, barnacles, tubeworms, hydroids, bryozoan, tunicates and diatoms while having a minimal bioaccumulation of toxicologically significant compounds [10,12]. Under normal use patterns, concentrations of 3–10% are sufficient. The encapsulation of booster biocides in microcapsules is an innovative technique that enables the controlled release of the biocide [2,13], as the biocide in the microcapsules acquires a pigment-like state. The biocide can be slowly released from the microcapsules’ surface and dissolved in the coating, after which it can diffuse through the coating to its surface. Encapsulation of booster biocides can also lower the environmental impact of the coating without compromising its antifouling properties. Laboratory tests on mussels indicated that encapsulation of booster biocides could potentially improve the antifouling efficacy of the biocides while decreasing their toxicity effects [2,14]. The reinforcement of paint matrix with nanoparticles can also improve the mechanical properties in the coating [15] such as tensile strength, flexibility and impact resistance. The common technique used for the measurement of mechanical strength is the impact test, performed according to ASTM D 2794 [16]. However, traditional methods need bulky standard samples to evaluate the mechanical behaviour of materials and as a consequence are not applicable in this case [17]. Moreover, in contrast to homogeneous materials, nanocomposite materials are much more complex in their microstructure and mechanical performance, as well as being time-dependent in their load response [18]. Therefore, advanced mechanical testing techniques, such as nanoindentation have become a commonplace tool for the measurement of mechanical properties at sub-micron scale [18,19]. The method developed by Oliver and Pharr allows determining the hardness and the elastic modulus from the nanoindentation load–displacement data [20]. The indentation response in the form of force-penetration (P-h) curves is characterized by two elastic constants, indentation modulus (through stiffness) and indentation hardness. Direct application of these equations to heterogeneous materials poses several difficulties, as the underlying analysis relies on the self-similarity of the indentation test which holds only for homogeneous materials. The interaction of phases in heterogeneous materials is unavoidable but depending on the length scale it can be more or less important. Properties extracted from indentation data of a heterogeneous solid can be treated as averaged quantities dependent on the depth [21]. Therefore, the choice of an indentation depth directly determines the length scale of the tested
2. Materials and methods 2.1. Microencapsulation of the booster biocide “SeaNine 211” Polysaccharide-based encapsulation technology was used for the preparation of the modified antifouling coatings, which were provided by Fraunhofer Institute for Applied Polymer Research IAP. Specifically, complex coacervation has been chosen as the suitable encapsulation technique [24]. The technology is based on the complex formation between a protein and oppositely charged polysaccharide and is already exploited in a few application fields on the industrial scale [24,25]. After the reaction of complex formation is completed, the system exists in the form of an aqueous suspension of hydrogel micro particles enclosing liquid core material (immiscible with water) [13]. The mechanical stability and the barrier properties of the capsule shells are comparatively poor at this stage. After drying, the shell of the particles becomes hard (but fragile), while the barrier properties of the shell are considered to be very good. Poor mechanical stability of the shell can be considered to be the main disadvantage of the technology [13]. The materials provided by Fraunhofer IAP were designed to have aninner supporting wall with good mechanical stability and simultaneously good permeability towards SeaNine, and an outer polysaccharide-based layer with suitable barrier properties. In total, four different encapsulations were tested in: gelatin and gum arabic (GGA) [26,27], polyurethane (PU) [28], melamine-formaldehyde (MF) [29] and a combination of PU and GGA (double shells) microcapsules [30]. The detailed synthesis description of the microcapsules in not in the scope of this paper, thus the relevant literature references have been provided for each of the microcapsules used in this study.
2.2. Coatings preparation SeaNine booster biocide was encapsulated in the four aforementioned types of capsules, which were mixed into tested standard selfpolishing coating formulation (Test product), provided by JOTUN S.A., containing no biocides. These new capsule-based paint formulations were applied on PVC plates using a paint applicator with a 400 μm gap size. The content of microcapsules in the Test product was adjusted to correspond to 2 wt.% of the active in the final coating. According to this, initially a formulation containing the active in a dissolved state was prepared (FSI). Then, GGA/SeaNine capsules were incorporated in the Test product at 6 wt.% (FSII), PU/SeaNine capsules at 6 wt.% (FSIII), PU/GGA/SeaNine capsules at 8 wt.% (FSIV) and finally MF capsules at 6 wt.% (FSV). Coating formulations containing no biocide at all were also prepared for comparison (Control sample).
2.3. Physical conditions The physical performance of the panels was visually assessed before immersion following international guidelines [31]. The coatings were visually inspected for cracking, peeling, blistering, wearing and pores or holes. Coatings displaying a smooth surface were judged as optimal. 2
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The coated PVC panels were immersed in the central-south Mediterranean waters in an industrial harbour of Malta (35°54′31.5″N; 14°23′34.0″E) from June 2015 until March 2016. The panels were fastened on a fibreglass board fixed to a marina quay and submerged at 1.5–3 m depth. The seawater temperature, pH, salinity, conductivity and dissolved oxygen concentration were monthly monitored with a portable meter Hach, HQ30d.
performed using the statistical package R [36]. Before analysis, the different sets of data were checked for assumption of normality and equality of variance using the Kolmogorov-Smirnov one-sample test and the Levene test, respectively. Wherever both assumptions were met, data were analysed by one-way ANOVA followed by Tukey HSD post hoc test. Data was analysed by the non-parametric Kruskal-Wallis test followed by the Nemenyi post-hoc test whenever either of the assumptions was not met. Differences were considered significant at p < 0.05.
2.5. Antifouling efficacy
2.6. Surface characterization under optical microscopy
2.5.1. Antifouling efficacy parameter At the end of immersion time, the panels were retrieved and gently rinsed with seawater from the site to remove excess silt [32]. A semiquantitative antifouling efficacy analysis of the coatings was carried out [32,33]. Aquadrant with 8 central divisions and a delimitation of 1 cm from the panel’s edge was placed over each panel to help in the visual assessment and recording of the estimated percentage of colonized surface. The antifouling performance of the tested paints was calculated using the Antifouling Efficacy parameter – “N parameter” as described by Bressy et al. [33]. The formula combines the composition, type of fouling and percentage of area covered:
The surface of each coating was analyzed through light microscopy (reflectance mode, dark field and polarised filters, magnification x100), using an AxioImager.A2m microscope with Zeiss lens. Three different points of the surface of each sample were examined. Special attention was given to defects and points of interest.
2.4. Immersion site
N=
∑ (IF
2.7. Nanomechanical properties measurements A Hysitron Tribolab® Instrument, equipped with a standard threesided pyramidal Berkovich tip, was used for the nanomechanical tests. Displacement resolution (0.04 nm) and load resolution (1 nN) at the vertical direction were high enough to allow detailed characterization of nanomechanical properties. Nanomechanical testing was applied in a clean area environment with 45% humidity and 23 °C ambient temperature. A standard fused silica sample was used to calibrate the area function of the Berkovich indenter tip before the application of measurements. All indentations were obtained with a single load-unload scheme, under Displacement Control (DC) mode; at DC mode the indenter tip is inserted into the sample surface in a predefined depth with simultaneous recording of the applied load. In the case of the present work all indentations were made at maximum depth of 200 nm. This depth was high enough to achieve an elastoplastic contact, without exceeding the 1/10th of the upper coating layer; as a result, plastic deformation was limited at the upper coating layer, i.e. from which nanomechanical data were extracted. Each indentation was made with a load and unload ramp of 40 and a dwell time at the maximum load of 3 s.
× SF )
Where IF is the intensity factor related to the percentage of cover for each type of organism and SF is the severity factor for each type of organism (SFslime = 1 and SFEncrusting animals = 6). 2.5.2. Fouling coverage A photo record of the test panels was also made with a camera Canon IXUS 155. The camera settings and focal distance were kept constant through time. Focal distance consistency was achieved using a support to hold the camera. The image processing software Image J [34] was used to estimate the percentage of area covered by the fouling community. Digital segmentation [1] of the images - isolation of the background colour from the components of interest- was used. 2.5.3. Wet weight The panels were placed in a plastic bag with seawater from the site and transported in a cooled box. In the laboratory, panels were placed vertically to drain the excess water for 3–6 min and weighted. Biomass was estimated as the difference between the wet weight of the panel covered with fouling and the initial weight of the panel. The results are expressed as the weight of the area covered by fouling community (mg/cm2) [3].
3. Results and discussion 3.1. Field immersion trials Throughout the experiment the temperature fluctuated between 15.7–27.7 °C (see Table 1) pH and salinity remained fairly constant. The physical condition of the control, FSI, FSIII, FSIV and FSV coatings were visually smooth before and after one month of immersion (see Table 2). FSII displayed a grainy texture.
2.5.4. Chlorophyll a content Fouling was scraped from panels on three squares of 1 cm2 with a spatula and transferred into falcon tubes (one square with heavy fouling, one with mild and one with little fouling). Samples were extracted in 90% acetone HPLC grade at 8 °C for 12–16 h in the dark, followed by centrifugation for 10 min. Chl–a was determined using spectrophotometry following the method described by Aminot and Rey [35], using a Thermoscientific spectrophotometer Genesis 10. The formula was adapted as follows:
3.2. Semi-quantitative antifouling efficacy After 9 months of exposure, the control coatings were heavily fouled, mainly by calcareous tubeworms, encrusted and branchy Table 1 Environmental parameters of the Mediterranean seawater throughout the field trial.
Chl–a = (11.85 × (E 664 − E 750) − 1.54 × (E 647 − E 750) Ve − 0.08 × (E 630 − E 750)) × ×L S Where: L = Cuvette light-path in cm, Ve = Extraction volume in mL, S = Surface area scraped in cm2. Concentrations are in unit mg/cm2. 2.5.5. Statistical analysis Statistical techniques to determine significant results were 3
Parameter
Average ( ± sd)
Min
Max
Temperature (°C) Salinity (‰) Conductivity (mS/cm) pH O2 (mg/L) Visibility (m)
21.63 ( ± 5.11) 39.29 ( ± 2.29) 55.58 ( ± 7.48) 8.10 ( ± 0.07) 7.95 ( ± 1.14) 2.83 ( ± 0.83)
15.70 35.40 44.70 8.01 6.48 2.00
27.70 41.70 64.50 8.22 9.50 4.00
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Table 2 Antifouling performance of coated panels immersed for 9 months in the Mediterranean Sea. N = 3. Significant differences between treatment and control groups are shown with *. Significant differences between coatings with microcapsules and FSI are indicated with +. Sample
Biomass ± sd (mg/ cm2)
Chlorophyll a ± sd (mg/ cm2)
Antifouling efficacy N ± sd
Fouling coverage ± sd (%)
Physical condition before immersion
Test Product without biocides (Control)
114.97 ( ± 4.93) 62.76 ( ± 24.09) 35.70 ( ± 6.98)* 54.81 ( ± 13.30) 76.09 ( ± 19.3) 80.80 ( ± 48.90)
7.31 ( ± 0.78) 3.62 ( ± 0.58)*** 3.36 ( ± 0.59)*** 3.97 ( ± 0.32) 5.56 ( ± 0.93)+ 5.63 ( ± 0.71)+
39.33 ( ± 10.11) 34.00 ( ± 6.25) 6.33 ( ± 2.31)*** 29.33 ( ± 4.62) 26.00 ( ± 6.24) 23.67 ( ± 7.50)
98.03 ( ± 0.79) 93.34 ( ± 4.77) 88.97 ( ± 3.95)* 96.38 ( ± 1.31) 91.57 ( ± 5.03) 94.48 ( ± 3.43)
smooth
4 wt% SeaNine in Test Product (FSI) 6 wt% GGA_SeaNine capsules in Test product (FSII) 6 wt% PU_SeaNine capsules in Test product (FSIII) 8 wt% PU/GGA_SeaNine capsules in Test product (FSIV) 6 wt% MF_SeaNine capsules in Test product (FSV)
Significant differences between treatment and control groups are shown with *. The
***
smooth grainy smooth smooth smooth
correspond to Tukey HSD post-hoc test with p = 5.9 × 104.
Fig. 1. Monthly progression of colonization by fouling organisms on coated panels in the Mediterranean Sea. Photographs of one replicate panel per treatment groups are displayed from first until last day of immersion.
FSIV (Tukey HSD post-hoc test, p < 0.032). The absence of encrusting organisms on FSII greatly decreased the N parameter since the severity factor of slime is 1.
bryozoan, barnacles, bivalves and green algae as seen in Fig. 1. Visual observation indicated the control coatings were colonised by a higher quantity of fouling organisms but also displayed a superior specie richness compared with the biocidal coatings (FSI-FSV). The percentage of hard fouling organisms (i.e. barnacles, bivalves, calcareous tubeworms, encrusting bryozoan) was greater on the control coatings than on the biocidal coatings as revealed by the semi-quantitative antifouling efficacy analysis (see Fig. 2). The control coating was mainly colonized by hard fouling organisms (> 50%) and displayed little bare space (Figs. 1 and 2). FSI, FSIII and FSIV were colonized by slime and hard fouling. FSV was colonized by slime and soft fouling while FSII displayed mainly slime. The N parameter, based on semi-quantitative observations, was the highest on the control coatings and the lowest on FSII (Table 2). No significant differences were identified between the N parameter of the control and FSI, FSIII, FSIV or FSV coatings (Tukey HSD post-hoc test, p > 0.1). On the other hand, FSII significantly decreased the N parameter compared to the control coatings (Tukey HSD post-hoc test, p = 5.9 × 10−4). FSII coatings also showed a lower N parameter compared to FSI, FSIII or
3.3. Quantitative antifouling efficacy The highest fouling coverage (98.03 ± 0.79) was observed on the control coatings while FSII displayed the lowest coverage (88.97 ± 3.95, Table 2). No significant differences were found between the fouling coverage percentage of the control and FSI, FSIII, FSIV or FSV coatings (Nemenyi post-hoc test, p > 0.19). On the other hand, FSII displayed a lower fouling coverage compared to control coatings (Nemenyi post-hoc test, p = 0.043). FSII coatings accumulated significantly less fouling biomass compared with the control coatings (Tukey HSD post-hoc test, p = 0.018). No significant differences were observed between the fouling biomass accumulated on FSI, FSIII, FSIV or FSV and the control coatings (Tukey HSD post-hoc test, p > 0.08). Similarly, no significant differences were found between FSII, FSIII, FSIV or FSV and FSI (Tukey HSD post-hoc test, p > 0.7). 4
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Fig. 2. Average percentage fouling coverage on coatings by category after 9 months of immersion in the Mediterranean Sea. N = 3.
curves (Fig. 3). The assessment of the nanomechanical properties is based on comparisons with the control sample, as the reference one. The control sample showed relatively good mechanical properties, presenting also the smoothest surface, which was reflected on the absence of outliers, while all load-displacement curves (except one) did not contain any discontinuities (see Fig. 3). FSI sample showed increased Er; however, the measured hardness was lower. The measurements produced also relatively more outliers that are possibly connected with the addition of SeaNine in pure state, which may cause “softening” points on the samples surface. FSII sample presented the lowest mechanical properties of all coatings. FSIII sample showed higher Er and H values, with the surface being rougher than the control, and a number of outliers between the control and FSI. FSV, as FSII, showed also lower values of Er and H than the control, and in parallel, both depicted high surface roughness and the highest number of outliers. At this point, it should be mentioned that the number of outliers indicates the inhomogeneity of the coatings surface, since the measurements on specific points was impossible. This can be attributed to the holes, pores and craters that were observed on the surface of the coatings.
FSI, FSII and FSIII displayed significantly less chlorophyll a, compared to control coatings (Tukey HSD post-hoc test, p < 3 × 10−4). Significantly more chlorophyll a was found on FSIV and FSV compared with FSI or FSII (Tukey HSD post-hoc test, p < 0.04). Overall, FSII displayed the most optimal antifouling efficacy compared to control coatings and also to FSI. FSIII displayed good antifouling efficacy while FSIV and FSV displayed limited antifouling efficacy. 3.4. Assessment of exposed surface Prior the nanomechanical testing, all samples were examined under optical microscopy in order to detect any possible defect that may affect the results. After the antifouling assessment, the specimens were rinsed with water and carefully cleaned, in order to remove any slime and residues of the fouling organisms. Representative images of the coatings surface are presented in Table 3. The control sample presents a smooth and homogenous surface with fading on several places. On the other hand, on FSI sample residues of microorganisms were detected (in black colour), as well as severe fading. Sample FSII presented a surface full of holes and craters of about 200 μm. Agglomerates also of the GGA capsules were detected in a yellowish jelly state. FSIII sample’s surface was full of PU particles, forming a porous and loose surface. Holes around 200 μm were also observed. On this sample, residues of fouling organisms (possible algae) were also detected in a linear morphology. FSIV sample presented the most heterogenous surface, full of particles similar to those observed on sample FSIII. Huge craters with a diameter of more than 760 μm were also detected. Finally, sample FSV had the most homogenous surface among the modified coatings with only some pores of ˜200 μm on some places. The morphology of the surface was quite close to the control’s sample. From the assessment of the exposed surface of all specimens it may be remarked that PU capsules form agglomerates. Also, all modified coatings presented increased porosity in respect to the control sample and FSI.
3.6. Overall comparative assessment The comparative results are presented in Fig. 6 and Table 5 below, which summarises the antifouling and mechanical properties, trying to classify the performance of all coatings, in terms of antifouling efficacy and mechanical integrity. Control sample without any biocide presented the worst antifouling performance, as expected, revealing on the other hand good mechanical properties, which were considered as the basis of the assessment. For the case of FSI, due to the presence of SeaNine in pure form, a 50% reduction of the detected chlorophyll was observed in comparison with the control sample. This sample proved to have also increased stiffness (8.51 μN/nm compared to 6.29 μN/nm of the control), which affects the leaching layer of the coating. The stiffer the coating is, the harder to self-polish. This means that the hydrolysis rate will be lower and as a result, the overall antifouling performance is not as high as expected. FSII presented the best antifouling performance for all endpoints. However, this coating may fail in the long term, since the release/ polishing rate is intense, considering the downgraded mechanical properties. Specifically, this sample presented the highest plastic
3.5. Nanomechanical properties results Samples that have been exposed in the field were examined with the nanoindentation technique. Table 4 includes the hardness values (H) and reduced modulus values (Er) extracted from the load-displacement 5
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Table 3 Surface assessment of tested samples. Dark field
Sample
Polarised field
Test Product without biocides (Control)
4 wt% SeaNine in Test Product (FSI)
6 wt% GGA_SeaNine capsules in Test product (FSII)
6 wt% PU_SeaNine capsules in Test product (FSIII)
8 wt% PU/GGA_SeaNine capsules in Test product (FSIV)
6 wt% MF_SeaNine capsules in Test product (FSV)
deformation which is calculated from the measured hardness, according to [37]:
Plastic Deformation = 1 −
However, the increased plastic deformation which is correlated with the self-polishing performance, indicates that the coating will be rabidly eroded, losing its effectiveness. On the other hand, FSIII exhibited good antifouling performance, with high mechanical integrity and
h max − hc h max
Table 4 Assessment of the data obtained from the nanoindentation testing. Sample
Er ± sd (GPa)
H ± sd (MPa)
H/E ± sd
Outliers
Test Product without biocides (Control) 4 wt% SeaNine in Test Product (FSI) 6 wt% GGA_SeaNine capsules in Test product (FSII) 6 wt% PU_SeaNine capsules in Test product (FSIII) 8 wt% PU/GGA_SeaNine capsules in Test product (FSIV) 6 wt% MF_SeaNine capsules in Test product (FSV)
7.12 ± 2.48 12.61 ± 9.03 0.47 ± 0.28 9.65 ± 0.84 2.05 ± 0.76 2.65 ± 0.28
835.4 ± 271.9 600.7 ± 387 21.9 ± 15 965 ± 91.7 235.3 ± 210.3 175.2 ± 57.4
0.12 0.06 0.04 0.10 0.12 0.07
0 3 3 2 1 3
For comparison reasons, the extracted values are depicted in bar charts form, in Figs. 4 and 5, respectively. 6
± ± ± ± ± ±
0.02 0.03 0.01 0.01 0.06 0.01
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Fig. 3. Characteristic load-displacement curves.
4. Conclusions
robustness, observed through load-displacement curves homogeneity (from Fig. 3 it can be noticed that the curves are not spread, but close to each other). Moreover, the increased elastic deformation, calculated also from the measured hardness, according to [37]:
Elastic Deformation =
This study aimed at a correlation between antifouling performance and nanomechanical properties of antifouling coatings containing loaded microcapsules. The results of the antifouling assessment have shown that the performance of the microcapsules made from different material combinations, can vary from the one coating system to the other. Since all coatings contain the same amount of biocide (active ingredient), it can be concluded that encapsulation of the biocide does allow the control of the release rate and as a result the antifouling efficacy. This was connected with the measured mechanical properties of the coatings, specifically the elastic and plastic deformation, as well as the stiffness and resistance to wear -connected to the release/erosion rate-. The slowest release was observed from melamine-formaldehyde microcapsules, and polyurethane/gelatine-gumarabic double walled
h max − hc h max
indicates a good functionality maintenance. This sample presented also the highest stiffness, with a low standard deviation. FSIV exhibited reduced antifouling performance simultaneously with a high wear resistance, proved by the high H/E ratio [38]. Finally, FSV presented the worst antifouling and mechanical performance compared to the rest coatings, indicating that the addition of MF capsules deteriorates the properties of the paint. 7
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Fig. 4. Coatings hardness.
Fig. 5. Coatings Young’s modulus.
Fig. 6. Comparative graph of mechanical and antifouling performance. 8
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Poor Performance).
Plastic Deformation
microcapsules which showed similar antifouling performances. Polyurethane microcapsules did not impact the release rate and the mechanical properties were similar with the coating containing the biocide in a free state. Gelatine-gum arabic microcapsules showed the fastest release, resulting in the highest antifouling performance, without expecting however its long-term efficacy. From the above it may be concluded that a compromise between mechanical and antifouling performance is necessary when considering the overall functionality of the coating. Additionally, the surface characteristics play also a crucial role, since the porosity and roughness of the surface affect the release of the biocide and the attachment of the fouling organisms, respectively. Assessing mechanical as well as antifouling properties of coatings helps predicting their long-term efficacy.
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[1] S. Dobretsov, J.C. Thomason, D.N. Williams, Biofouling Methods, John Wiley & Sons, Ltd., Oxford, UK, 2014, https://doi.org/10.1002/9781118336144 ISBN 9781118336144. [2] E. Gutner-Hoch, R. Martins, T. Oliveira, F. Maia, A. Soares, S. Loureiro, C. Piller, I. Preiss, M. Weis, S. Larroze, T. Teixeira, J. Tedim, Y. Benayahu, Antimacrofouling efficacy of innovative inorganic nanomaterials loaded with booster biocides, J. Mar. Sci. Eng. 6 (1) (2018) 6, https://doi.org/10.3390/jmse6010006. [3] I. Menchaca, I. Zorita, N. Rodríguez-Espeleta, E. Erauskin, C. Erauskin, P. Liria, M. Santesteban, I. Mendibil, I. Urtizberea, Guide for the evaluation of biofouling formation in the marine environment, Rev. Investig. Mar. 21 (2014) 90–99. [4] R.A. Braithwaite, L.A. McEvoy, Marine biofouling on fish farms and its remediation, Adv. Mar. Biol. 47 (2004) 215–252, https://doi.org/10.1016/S0065-2881(04) 47003-5. [5] C. Hellio, D. Yebra, Advances in Marine Antifouling Coatings and Technologies, (2009), https://doi.org/10.1533/9781845696313 ISBN 9781845693862. [6] 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, https://doi.org/10.1016/j.porgcoat.2003.06. 001. [7] C.M. Kirschner, A.B. Brennan, Bio-inspired antifouling strategies, Annu. Rev. Mater. Res. 42 (2012) 211–229, https://doi.org/10.1146/annurev-matsci-070511155012. [8] Woods Hole Oceanographic Institute Characteristics of Antifouling Coatings, Marine Fouling and Its Prevention, George Banta Publishing Co., 1952, pp. 313–322. [9] CEPE Function of Zinc Oxide in Antifouling Paints, (2010). [10] K.V. Thomas, S. Brooks, The environmental fate and effects of antifouling paint biocides, Biofouling 26 (2010) 73–88, https://doi.org/10.1080/ 08927010903216564. [11] L.D. Chambers, K.R. Stokes, F.C. Walsh, R.J.K. Wood, Modern approaches to marine antifouling coatings, Surf. Coat. Technol. 201 (2006) 3642–3652, https://doi.org/ 10.1016/j.surfcoat.2006.08.129. [12] L. Chen, J.C.W. Lam, SeaNine 211 as antifouling biocide: a coastal pollutant of emerging concern, J. Environ. Sci. 61 (2017) 68–79, https://doi.org/10.1016/j.jes. 2017.03.040. [13] R.L. Hart, D.R. Virgallito, D.E. Work, Microencapsulation of biocides and antifouling agents, Microtek Laboratories Inc. (Dayton, OH, US), USA, 2011. [14] F. Avelelas, R. Martins, T. Oliveira, F. Maia, E. Malheiro, A.M.V.M. Soares, S. Loureiro, J. Tedim, Efficacy and ecotoxicity of novel anti-fouling nanomaterials in target and non-target marine species, Mar. Biotechnol. 19 (2017), https://doi. org/10.1007/s10126-017-9740-1. [15] J. Dustebek, C. Kandemir-Cavas, S.F. Nitodas, L. Cavas, Effects of carbon nanotubes on the mechanical strength of self-polishing antifouling paints, Prog. Org. Coat. 98 (2016) 18–27, https://doi.org/10.1016/J.PORGCOAT.2016.04.020. [16] ASTM International, Standard Test Method for Resistance of Organic Coatings to the Effects of Rapid Deformation (Impact), ASTM International, 2015, pp. 12–14, https://doi.org/10.1520/D2794-93R10 06.01. [17] I.A. Kartsonakis, E.P. Koumoulos, A.C. Balaskas, G.S. Pappas, C.A. Charitidis, G.C. Kordas, Hybrid organic–inorganic multilayer coatings including nanocontainers for corrosion protection of metal alloys, Corros. Sci. 57 (2012) 56–66, https:// doi.org/10.1016/J.CORSCI.2011.12.034. [18] E.P. Koumoulos, D.A. Dragatogiannis, I.A. Kartsonakis, E. Karaxi, T. Kehagias, C.A. Charitidis, Nanomechanical performance of protective coatings reinforced
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Test Product without biocides (Control) 4 wt% SeaNine in Test Product (FSI) 6 wt% GGA_SeaNine capsules in Test product (FSII) 6 wt% PU_SeaNine capsules in Test product (FSIII) 8 wt% PU/GGA_SeaNine capsules in Test product (FSIV) 6 wt% MF_SeaNine capsules in Test product (FSV)
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Stiffness (μN/nm) Resistance to wear (H/E)
This work was supported by the EU FP7 Project “Low-toxic costefficient environment-friendly antifouling materials” (BYEFOULING) under Grant Agreement no. 612717. The authors would like to thank Jotun S.A. for providing the benchmarked coatings, as well as Dr. Alexandra Latnikova from Fraunhofer Institute for Applied Polymer Research IAP, for the modified coatings with the encapsulated biocides. References
Biofilm inhibition efficacy
Overall Antifouling efficacy
Acknowledgments
Coating
Table 5 Ranking of mechanical properties with antifouling efficacy of coatings exposed in the Mediterranean Sea for 9 months. (**** Optimal Performance,
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