The antifouling performance of gelcoats containing biocides and silver ions in seawater environment.
In this study, the recruitment and succession of biofilms were examined on static panels representing a boat hull exposed to seawater. Panels were coated with five types of gelcoats to compare the effect of antifouling agents on the forming of biofilms. Pesticide and elemental silver-based additives in two weight ratios (0.003 and 0.006) were used as the antifouling agents for the gelcoat. The temporal variation in recruitment of biofilms was examined by isolation of attached bacteria during a boat storing period in a marina located along the Izmir Bay (Turkey). It was found that the gelcoat with silver ion had slight superiority over the counterparts in this study. The first 15 days of bacteria formation was the significant period for examining the antifouling performance of a coating material.
Keywords Gelcoats in marine use, Antifouling characteristics, Biocides, Silver ion
Many of the bacteria in natural environments grow on surfaces, including liquid-solid and liquid-air surfaces, (1) forming biofilms. Microbial biofilms themselves can increase boat hull roughness, thereby increasing fuel consumption, and they may contribute to the attachment of shellfish larvae to the hull, (2), (3) which leads to markedly increased fuel consumption. (4), (5) To prevent fouling problems on surfaces immersed in water, antifouling paints can be applied to some surfaces. (6)
Three major approaches have been applied to render materials antimicrobial: antiadhesive, biocide-release, and contact-active antimicrobial modifications. Most commonly, bulk materials or coatings are loaded with biocides, such as silver ions, halogens, and antibiotics, which are then slowly released into environment, killing microbes there. (7)
Biocide-containing coatings are already used and applied to the hulls of ships and boats to prevent the growth of bacteria, macroalgae, mussels, and other invertebrates. Worldwide, around 18 compounds are currently used as antifouling biocides. (8)
An effective alternative method is the coating of surfaces with elemental silver. Elementary silver is believed to function antimicrobially, either as a release system for silver ions or as a contact active material. (7)
In this study, the recruitment and succession of biofilms were examined on static panels representing a boat hull exposed to seawater in the Izmir Bay (Turkey). Panels were coated with five types of gelcoats to compare the effect of antifouling agents on the formation of biofilms. Biocide and elemental silver-based additives in two weight ratios (0.003 and 0.006) were used as the antifouling agent for the gelcoat. The temporal variation in recruitment of biofilms was examined by isolation of the attached bacteria. Antifouling performances of these gelcoats were finally compared as the conclusion.
The basic product used in this study was an isophthalic/Neopentil glycol-based, acrylic-modified, thixotropic, pre-accelerated, high advance exterior use, brush-type gelcoat [(Polijel.sup.[TM]] F-215 of Poliya Poliester). In Table 1, the properties of this gelcoat are presented.
Table 1: Properties of the gelcoat used in the study Test Method Value Properties Viscosity ISO 2555 4000-8000 cp of liquid Brookfield[R] form (It is measured at 23[degrees]C, 50/5 rpm with spindle 6) Thixotropy (It Thixotropic is measured at 23[degrees]C by adding 1% mL Mek-p (Butanox M60)) Gel time ISO 2535 10'-15' Mechanical Heat ISO 0075-A 93[degrees]C properties deflection of cured temperature resin (HDT) (Before the Water ISO 0075-B 101[degrees]C Mechanical absorption ISO 0067 160% tests, Barcol ASTM-D 2583 48 the cured hardness resin is (Barcol post-c 934-1) Flexural ISO 0178 124 MPa strength Elongation at ISO 0178 4.4% break Tensile ISO 0527 68 MPa strength Elongation at ISO 0527 2.6% break
Two types of additives with two different weight ratios were added to the gelcoat by mixing to gain antifouling characteristics. With four antifouling gelcoats, and one control gelcoat without an additive, five coated panels representing boat hull surfaces were used in this comparative study. The ratios are given in Table 2.
Table 2: Contents of the gelcoats used in the panels Panel number Additive and its weight ratio 1 3[per thousand] (Chlorothalonil + triazine containing mixture) 2 6[per thousand] (Chorothalonil + triazine containing mixture) 3 3[per thousand] (Silver-ion-containing mixture) 4 6[per thousand] (Silver-ion-containing mixture) 5 No additive
Preparation of static panels
To provide suitable surfaces for the gelcoat applications, 30 cm x 30 cm square panels were manufactured by the contact method, more widely known as the hand lay-up method. To simulate bright and smooth boat skin, a plexiglass panel was implemented as the molding surface.
Since five different types of gelcoat were to be examined, five sets of panels were to be produced. Hence, an area on the molding surface corresponding to the total area of the complete set of panels was treated with a "carnauba" type of mold-releasing wax (Meguiar's[R] Mirror Glaze[R]). This wax film applied on a Plexiglas molding surface, or on any mold surface in general, prevents the sticking of gelcoats and subsequent lamination to the mold.
Although the panels were of no structural significance, for the sake of reliability of test results, lamination was carried out at the end of August and the beginning of September when optimum ambient conditions for the lamination process prevailed in the region where the workshop is located. At the time of lamination, the ambient temperature was 25[degrees]C and relative humidity was 35%.
Because the aim was solely to verify the chemical resistance of said gelcoats against fouling organisms, and to prevent any effects of pigments (favorable or adverse), it was decided to produce panels without pigmentation. In case some types of these gelcoats are to be used in future production, bubble formation or porosity of the gelcoats was to be observed qualitatively.
Providing cutting, trimming tolerances, and sufficient gap between panels, every type of gelcoat was applied by brush. Considering that panels were not to be subjected to any load with respect to stamina, only one layer reinforcement--i.e., 600 g/m square chopped strand mat--was laminated, yielding 1.5 mm thickness.
Because the internal surface of hand-laminated products is not smooth if left without further treatment, excessive fouling organisms on these faces should be anticipated. To eliminate this phenomenon leading to erroneous results, rough internal faces were applied with relevant gelcoats following smoothing of surfaces and trimming of edges.
One should note that gelcoats are "air-inhibited" materials. In other words, during polymerization, atoms at the end of polymer chain meet with atoms of oxygen in the air, and polymerization ceases. This means that a surface exposed to air does not cure completely, and stays soft and in a sticky state. Therefore, considering that the gelcoats applied to the internal face of panel were not close molded and would be exposed to air, they were treated with the addition of a 5% paraffin solution produced by Dewilux[R]. The paraffin used in the solution was of Grade 52 (melting point is 52[degrees]). Here, the paraffin solution did not participate in the chemical reaction, but moved to the surface due to exothermic behavior from lamination, and converted to a film over the surface. Thus, the encapsulating gelcoat helps its complete cure.
With the above-mentioned process, panels were obtained with both faces possessing identical physical and chemical properties, with the anticipation of uniform marine growth accumulation on both faces.
Static panels were submerged to a depth from the surface of 1.5 m in a marina located along the Inner Izmir Bay coast. Izmir Bay is a long embayment situated along the eastern Aegean Sea. It is naturally divided into three parts: Outer, Central, and Inner Izmir Bays. The Outer Izmir Bay is 20 km wide, between Karaburun and Foca and extends 45 km in a northwest--southeast direction. Uzun Island divides the Outer Izmir Bay into 6 km-wide western and 12 km-wide eastern segments. (9)
Panels were submerged for the time interval to cover the storing season of a boat in the marina (90 days). On the 7th, 15th, 30th, 60th, and 90th days, the panels were collected for analysis and then submerged again.
Physical and chemical parameters such as dissolved oxygen (Winkler methods), pH, temperature, and salinity were measured by Wissenschaftlich Technische Werkstatten (WTW). The pH/Cond 304i/set of the seawater during the test period are presented in Table 3.
Table 3: Values of physico-chemical parameters during September-December 2007 Sample period (Day) Dissolved oxygen pH Temperature Salinity (mg [L.sup.-1]) ([degrees]C) (psu) 1 4.99 8.12 24.9 40.4 7 4.30 8.09 23.8 40.9 15 4.85 8.13 23.2 40.5 30 6.02 8.20 18.4 38.3 80 6.67 8.13 13.1 38.9 90 6.99 8.16 11.0 38.7
The surfaces of the specimens were scraped and suspended in 10 [cm.sup.3] of filter-sterilized aged seawater using a cell scrapper (Corning). A dispersed and diluted sample was inoculated on ZoBell 2216e agar medium (peptone 5 g, yeast extract 1 g, Fe[PO.sup.4] 0.01 g, agar 15 g, distilled water 250 [cm.sup.3], aged seawater 750 mL, pH 7.2) and seawater-based R2A agar medium (proteose peptone 0.5 g, yeast extract 0.5 g, casamino acid 0.5 g, glucose 0.5 g. soluble starch 0.5 g, sodium pyruvate 0.3 g, [K.sub.2][HPO.sub.4] 0.3 g, Mg[SO.sub.4] 7[H.sub.2]O 0.05 g, agar 15 g, distilled water 500 [cm.sup.3], aged seawater 500 [cm.sup.3], pH 7.2), respectively. The agar plates were incubated at 25[degrees]C for 3 days. Counts of culturable bacteria were determined by the spread plate method. (10)
Analysis of variance (ANOVA) was used to compare the changes in the different panels by means of cultivatable heterotrophic bacteria, and then the Tukey Honesty nonparametric test was chosen for further analysis. The Pearson correlation procedure was also applied for physico-chemical and bacteriological parameters. (11)
A visual comparison can be made by considering the photos of the panels taken before submerging and at the end of sampling period (Fig. 1).
[FIGURE 1 OMITTED]
Total bacteria numbers in the unit area of 1 [cm.sup.2] and their changes over the six sampling periods are given. The first stage where the increasing bacteria numbers occurred was taken after almost 15 days. After this period, as the plate surfaces were coated by fouling organisms, the bacteria numbers attached to the surfaces of the panels decreased.
Throughout the study period, concentration of heterotrophic bacteria varied between 2.8 x [10.sup.5] and 1.1 x [10.sup.8] cfu/[cm.sup.2]. Maximum heterotrophic bacteria in panels were recorded on Panel 2, and minimum concentrations were recorded in Panel 5 (Table 4). The changes in cultivable heterotrophic bacterial densities (log) are shown in Fig. 2.
[FIGURE 2 OMITTED]
Table 4: Ranges and mean values of total viable bacteria on the surface of panels (cfu/[cm.sup.2]) Panel number Min. Max. Mean 1 9.6 x [10.sup.5] 2 x [10.sup.7] 6.5 x [10.sup.6] 2 3.2 x [10.sup.6] 1.1 x [10.sup.8] 3.6 x [10.sup.7] 3 5.2 x [10.sup.5] 2.6 x [10.sup.7] 1.3 x [10.sup.7] 4 3 x [10.sup.6] 3.6 x [10.sup.7] 1.2 x [10.sup.7] 5 2.8 x [10.sup.5] 1.6 x [10.sup.7] 8.2 x [10.sup.6]
According to one-way ANOVA result, the cultivable heterotrophic bacteria quantities did not display any significant change in different panels through the sampling period (p < 0.05). This result was checked with Tukey Honesty test.
The relations between cultivable heterotrophic bacteria densities and other environmental parameters were also evaluated. A strong correlation between culturable heterotrophic bacteria densities and physico-chemical parameters was not found (Table 5).
Table 5: Correlation between heterotrophic bacteria and physico-chemical parameters (p < 0.05) Panel number Temperature Salinity pH Dissolved oxygen 1 -0.38 -0.23 -0.05 0.35 2 0.17 0.21 -0.01 -0.20 3 -0.41 -0.29 0.29 0.37 4 -0.37 -0.24 -0.13 0.31 5 -0.32 -0.31 0.26 0.30
In this comparative study, cultivable heterotrophic bacteria concentrations on the surfaces of panels submerged in seawater were measured for biocide-based and silver ion-based antifouling gelcoats used in the marine industry.
Bacteria concentrations on the surfaces of the plates rapidly increased over a period of the first 15 days. As other fouling organisms grew on the surfaces by coating them, the related concentrations were decreased after this period.
Although no significant effects of additives on the antifouling performance of the gelcoat were examined, it can be seen that the antifouling performance of the gelcoat with silver ion was improved by doubling the amount of additive, while no positive effect of the additive amount on the performance was observed for the gelcoat with pesticides.
Since the control panel had almost the same bacteria accumulation as the other panel, it can be concluded that the surface roughness caused by the additives provided suitable surfaces for the fouling organisms that prevented the biofilm from forming. No strong correlation between bacteria cultivatable heterotrophic bacteria concentrations and marine environmental conditions was found during the study period.
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G. Neser(*), A. Kacar
Dokuz Eylul University Institute of Marine Sciences and Technology, Baku B. 10 Inciralti, Izmir 35340, Turkey e-mail: email@example.com
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|Title Annotation:||BRIEF COMMUNICATION|
|Author:||Neser, Gokdeniz; Kacar, Ash|
|Date:||Jan 1, 2010|
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