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An improved laboratory reattachment method for the rapid assessment of adult barnacle adhesion strength to fouling-release marine coatings.

Abstract Modifications have been made to the previously described adult barnacle laboratory reattachment method to enhance and improve the overall utility of this technique for rapidly assessing the efficacy of novel fouling-release marine coating technologies. These modifications include the use of an immobilization template to secure barnacles onto the coating surfaces during the underwater reattachment process, the development of a semi-automated push-off device to enable consistent and reproducible force gauge measurements and the implementation of a software tool to measure the diameter of barnacle base plates for adhesion strength calculations. A series of experimental siloxane--polyurethane and control coatings were evaluated with both the original and improved laboratory reattachment methodologies. Significantly higher adhesion strengths were obtained on these coatings using the improved reattachment method. Furthermore, only the improved reattachment method was able to discern significant differences in the performance of the siloxane--polyurethane coatings based on differences in compositional components. In this regard, the siloxane--polyurethane coatings containing the poly-caprolactone end groups attached to the poly(dimethylsiloxane) (PDMS) backbone exhibited significantly higher reattached barnacle adhesion strengths than the aminopropyl-terminated PDMS containing coatings. It was also shown that the utilization of barnacles with 5-6 mm base plate diameters, rather than 7-8 mm diameters, significantly enhanced the strength or tenacity of adhesion to the surface of the control coatings. The results of the improved laboratory reattachment evaluation of experimental siloxane--polyurethane and control coatings were in good agreement with barnacle adhesion measurements obtained for the same coatings with static ocean immersion testing in the field.

Keywords Barnacle adhesion, Fouling-release, Barnacle reattachment, Marine coatings, Laboratory testing

Introduction

Barnacles are one of the most problematic and well-studied marine organisms associated with the fouling of man-made structures immersed in the sea. (1) The adult form of the barnacle can impose a significant hydrodynamic drag penalty when attached to the hull of a ship, leading to a substantial increase in fuel consumption during vessel operations. (2-4) Naval vessels and other intermittently used watercraft are extremely susceptible to barnacle fouling as they typically spend extended periods of time in port where barnacle fouling is most aggressive. (5) Once barnacles colonize the surface of the hull, these vessels must be periodically dry docked to remove them, resulting in increased maintenance costs and loss of operational deployment time.

The most commonly used approach to mitigate barnacle fouling on ship hulls has been to coat them with a conventional antifouling coating. However, the recent world-wide ban on the use of organotin containing coatings by the International Maritime Organization (IMO) and increasing environmental concerns over the use of copper-based coating technologies has mandated the need for the development of environmentally friendly and effective coating alternatives. (6), (7) The most promising alternative antifouling technology investigated to date has been the silicone-based fouling-release coatings. (8) These coatings rely on their ability to prevent or minimize the adhesion of marine fouling organisms to promote a "self-cleaning" mechanism of antifouling by the action of hydrodynamic forces imparted on the surface of the hull once the vessel attains sufficient operational speed. (2), (9) Although effective, current silicone-based coatings are mechanically weak and typically suffer from short effective lifetimes. (7), (10) As a result, a considerable amount of effort and resources has been leveraged over the past few years to improve on the inadequacies of current fouling-release coating technologies."

It is widely accepted that ocean immersion testing is one of the most effective means to evaluate the performance of new fouling-release marine coating technologies. Ocean immersion testing can provide marine coating developers an abundance of valuable information, such as the broad spectrum performance of a coating against a wide range of fouling organisms and its long term durability in a challenging, real-world environment. However, several aspects of ocean immersion testing can constrain or impede the progress of marine coating development efforts, including limited testing capacity, large material requirements, length of testing time, and the seasonality of certain field testing sites. These limitations do not necessarily preclude the testing of new coatings in the field, but merely serve as key factors to be considered when developing a new antifouling marine coating technology(s). In certain instances, it may be advantageous to supplement ocean immersion testing with an appropriate laboratory-based methodology to alleviate some of the impediments encountered with performance evaluations in the field. (12)

The authors have previously reported on the development or a novel adult barnacle laboratory reattachment method to accelerate the performance evaluation of experimental fouling-release marine coatings. (11), (13-18) This testing methodology offers some unique advantages to traditional laboratory evaluation techniques employing barnacles as it (1) allows for experimental coating compositions to be analyzed in as little as 2 weeks time when supplied with an adequate number of adult barnacles, (2) does not require the coatings development team to acquire the expertise needed to culture and maintain larvae or spat, and (3) alleviates the problems associated with the settlement and rearing of cyprids on coating surfaces that exhibit some degree of latent toxicity in a laboratory setting. In addition, the laboratory reattachment method can be used as an efficient and effective means to quickly evaluate and downselect large arrays of experimental coatings generated with a combinatorial, high-throughput approach for more advanced and rigorous performance evaluations, such as static ocean immersion testing. (17) In fact, when supplied with a sufficient number of adult barnacles (~ 1000 per month supplied by Duke University Marine Laboratory (DUML) to North Dakota State University (NDSU)), as many as 100 unique coating compositions can be evaluated each month at NDSU.

The following report describes modifications that have been made to the original adult barnacle laboratory reattachment methodology to improve the overall utility of the reattachment technique for rapidly assessing the performance of fouling-release marine coatings. In addition, a greater amount of detail has been included in this report to enhance the usefulness of this testing methodology for other research groups developing or evaluating the performance of new fouling-release coating technologies in a laboratory setting.

Experimental

Coating preparation

International Intersleck 425[R] (IS 425) and 900[R] (IS 900), Dow Corning Silastic T-2[R] (T2), and an in-house polyurethane coating (POLY) were prepared on epoxy-primed stamped 4" x 8" marine., grade aluminum panels as described previously (15), (18) The coatings were prepared to achieve a dry film thickness of [greater than or equal to]350 [micro]m to minimize the influence of coating thickness on adhesion strength measurements. (19) A library of crosslinked siloxane--polyurethanc fouling-release coatings were prepared as draw downs on epoxy-primed 4" x 8" marine grade aluminum panels as described previously. (20) Specifically, a subset of this coating library was prepared for this study and the compositional details of each coating evaluated are provided in Table 1. All coatings were immersed in a re-circulating fresh water (city tap water) tank for 14 days to precondition the coating surfaces and facilitate the removalleaching of any toxic impurities (i.e., residual catalyst, monomers, solvent, etc.) that may be present after the coating preparation process. Adult barnacles (Amphibalanus (= Balanns) amphitrite (21)) were reared at the DUML as described previously. (15) Briefly, barnacle nauplii obtained from field-collected adults were reared in mass culture to the cypris or settlement stage, on Skeletonema costatum. Cyprids were collected by a sieve cascade from cultures after 4 days, cleaned of debris and held at 6[degrees]C for 1-3 days and then settled on Silastic T-2[R] glass panels in drops. Settled barnacles were reared by placing 2 T-2 panels in a plastic food container (8 x 8 x 14 cm) containing 725 mL of aged filtered seawater. Water in the containers was changed twice per week. Each day for the first 4 weeks, barnacles were fed a mixture of 20-40 mL of Skeletonenia costatum and Dunaliella tertiolecta. After 3 weeks, 10 mL of 1-clay-old brine shrimp nauplii Artemia spp., hatched from a stock of four teaspoons of Sanders[R] Great Salt Lake Artemia Cysts in 4000 mL of 30 salinity water, was added to each container daily. After 6 weeks, the primary food was 10 mL of 1-day-old brine shrimp nauplii per day.
Table 1: Siloxane--urethane coating compositions

Coatng            PDMS Mn         PCL block
                  (g/mol)         length

PU-3               20,000                 0
PU-4               30,000                 0
PU-PCL-3           20,000                 3
PU-PCL-4           30,000                 3

PDMS poly(dimethylsiloxane), Mn molecular weight,
PCL poly(caprolactone)


Once the barnacles reached a sufficient size for testing (>5 mm in base plate diameter) on the Silastic T-2[R] glass panels, they were shipped overnight to NDSU. The panels were wrapped in moist paper towels, sealed in a plastic bag and placed in a cooler to maintain the barnacles at suitable conditions to survive the transit. Upon arrival at NDSU, the panels of adult barnacles were removed from the cooler and immediately placed in an aquarium tank system with electronic monitoring and maintenance of pH (8.1 [+ or -] 0.1), salinity (33.7 [+ or -] 1.3 ppt), and temperature (23[degrees] [+ or -] 2[degrees]) of the re-circulated artificial sea water (ASW). A 10% change in total volume of the aquarium system was completed each day with fresh ASW.

Reattachment procedure

The reattachment of adult barnacles to coating surfaces using the original method was carried out according to the procedure reported by Rittschof et al) (15) Briefly, adult barnacles of suitable size (5-8 mm base plate diameter) were dislodged from the T2 rearing substratum, measured for base plate diameter using calipers, dried with a paper towel and placed immediately on the coating surfaces to be evaluated. As shown in Fig. 1, only barnacles that had a flat and intact base plate were selected for reattachment to a coating surface. Adult barnacles showing any visible signs of shell breakage, cracking, or "cupping" of the base plate were excluded from this study. Coating surfaces with newly placed barnacles were then housed in 22 x 14 x 5 cm plastic containers for 3 h and then transferred to an empty aquarium tank. Re-circulated ASW was slowly added to the aquarium tank and then the flow rate was increased to normal operational settings. Each aquarium tank was aerated during the entire period of reattachment. Barnacles undergoing reattachment were fed daily with 400 mL of 1-day-old brine shrimp nauplii by terminating the flow of ASW for 4 h and increasing the aeration (i.e., maximum air flow) to ensure adequate mixing and distribution of nauplii throughout the entire volume of the aquarium tank.

[FIGURE 1 OMITTED]

A duplicate set of coatings was also evaluated using the improved laboratory reattachment methodology. Once adult barnacles of suitable size (5-8 mm base plate diameter) were placed on the coating surfaces, as described in the original method above, a custom-designed immobilization template was applied to each coating panel to secure the barnacles in place during the entire reattachment process (Fig. 2). The immobilization templates were fabricated from 3/16" sheets of polyvinyl chloride (PVC) using a CNC milling technique. Once machined, the templates were sanded on one side, with 200 grit sandpaper, sprayed with 3 M Super 77[TM] multipurpose adhesive and allowed to dry for 5 min. The "tacky" templates were then placed adhesive side down onto an elastic nylon 22 mesh fabric (swimsuit lining) and allowed to dry for several hours. The mesh fabric was stretched on a large frame prior to adhesion to the PVC frame to remove any wrinkles and to create a slight degree of tension. The added tension or "preload" within the mesh fabric improves the template's ability to effectively immobilize the reattached barnacles. After drying of the adhesive (24 h), the excess mesh fabric was trimmed from the immobilization templates.

[FIGURE 2 OMITTED]

The immobilization templates were designed to reattach barnacles to either combinatorial coating arrays or fully coated panels (4" x 8") and to accommodate barnacles of different shapes and sizes by punching holes of two different diameters (1.6 and 2.4 mm) into the elastic nylon mesh fabric. The holes were incorporated into the mesh fabric to allow the immobilized barnacles to feed on brine shrimp nauplii during the entire duration of reattachment. It is important to note that the holes cut into the mesh fabric make it a requirement that all barnacles must be [greater than or equal to]5 mm in base plate diameter to be properly immobilized to the coating surfaces. Figure 3 demonstrates the process carried out to ensure that each barnacle is properly immobilized by using a straight dissecting needle to adjust the mesh fabric so that it fits appropriately over the shell and applies adequate downward pressure on the barnacle towards the coating surface. Elastic rubber bands or plastic zip ties were used to secure the immobilization templates in place and to prevent any movement when being placed in the aquarium tanks and immersed in artificial seawater. In contrast to the original reattachment methodology which requires a 3-h initial attachment period in plastic containers, coating panels were immediately transferred to aquarium tanks and immersed in seawater to facilitate initial attachment under water. Barnacles reattached with the immobilization templates were kept under the same conditions as those reattached with the original method described above. including daily feedings of brine shrimp nauplii.

[FIGURE 3 OMITTED]

An additional set of IS 425, IS 900. T2. and KJ coatings were also evaluated with the improved laboratory reattachment method, but with barnacles of different base plate diameters. Specifically, nine barnacles with a base plate diameter of 5-6 mm and nine barnacles with a base plate diameter of 7-8 mm were placed on the same replicate panel for each of the four control coatings. Subsequently, the barnacles were secured in place with the immobilization template and reattached using the same procedure described above for the improved laboratory reattachment method.

Measurement of barnacle adhesion in shear

The measurement of barnacle adhesion in shear was carried out by following the method described in ASTM D5618-94. (22) For the coatings evaluated with the original reattachment method, adult barnacles 5-8 mm in base plate diameter were exposed to a force applied to the edge plate at the base parallel to the substratum using a handheld mechanical force gauge until the barnacle detached from the surface. Adhesion strength was based only upon barnacles that gave measurable detachment forces (>0 Ibs). If after detachment, any visible amount of the barnacle's base plate remained on the surface, the barnacle was discarded and the data was not used in the adhesion strength calculation. Otherwise, the diameter of the barnacle was measured with calipers and the base plate area estimated. Adhesion strength was calculated by dividing the measured force required to remove the barnacle by the base plate area. Adhesion strength measured in this way is typically independent of the attachment area of the barnacle. (23)

For the coatings evaluated with the improved reattachment method, adult barnacles 5-8 mm in base plate diameter were exposed to a force applied to the edge plate at the base parallel to the substratum using a handheld mechanical force gauge mounted on the deck of a motorized stage (Fig. 4). Similar to other studies in the literature, the motorized stage was designed to provide a consistent and repeatable platform on which to measure the adhesion strength of reattached barnacles. (19), (24) This semi-automated push-off device serves to both fix the angle of the force gauge and to apply a constant push-off rate that is consistent and independent of the operator. The device contains an adjustable sample holder that can accommodate a variety of substrate sizes and geometries. Threaded height adjusting knobs and a fish-eye level are used to ensure that the force gauge is applied to the base of the reattached barnacles at a consistent height and angle for every barnacle attached to the array of coatings being evaluated. The force gauge itself is mounted on a sliding deck that is driven by a lead screw at 1.5 mm/s towards the base of the reattached barnacles. The sliding deck is directionally controlled via a toggle switch mounted on the side of the device. The entire unit was constructed out of aluminum and nylon to minimize the corrosive action of the artificial sea water associated with the barnacle reattachment procedure.

[FIGURE 4 OMITTED]

The original reattachment method required the use of calipers to measure the base plate area of each barnacle pushed off of the control and experimental coating surfaces. For coatings evaluated with the improved reattachment method, a commercially available software package was used to simultaneously measure the base plate area of each barnacle used in the adhesion evaluations. Specifically, each barnacle pushed off a coating surface was placed on the surface of a flat bed scanner, organized by the specific coating from which they were pushed off, and scanned to obtain a digital image. A 47 mm filter membrane was also included in the digital image for use as a scaling reference. To ensure accurate measurement of barnacle base plate areas were obtained, the scanner settings were adjusted to achieve good contrast between the barnacle base plates and the background (Fig. 5). Calculation of barnacle base plate diameters and areas were carried out using the software package Sigma Scan Pro 5.0. In this regard, the 47 mm filter membrane was first used to calibrate a standard distance by performing a 2 Point Resealing (Image [vector] Calibrate [vector] Distance and Area [vector] 2 Point Resealing) on the filter membrane, in which two points on opposite edges of the membrane were selected and the "New Distance" value was entered as '47'. In order to calculate the diameter and area of the reattached barnacle base plates, "Feret Diameter" and "Area" were selected as measurement types (Measurements [vector] Measurement Settings [vector] Measurements [vector] Area and Feret Diameter). The image thresholding was then adjusted to ensure proper detection and discrimination of the base plate pixels from the background pixels (Measurements [vector] Measurement Settings [vector] Fill [vector] Auto Thresholding [vector] select "Lighter then Background" and enter '50' in the % box). The "Fill" function was then selected to calculate the Feret diameters and areas by clicking the center of the 47 mm filter membrane and each individual barnacle to he measured. This effectively fills each item being measured with red pixels (Fig. 5) and yields a spreadsheet providing the calculated Feret diameter and area values for each reattached barnacle.

[FIGURE 5 OMITTED]

Static ocean immersion testing

Florida Institute of Technology

The siloxane--urcthane coatings and IS 425 coated on 4" x 8" aluminum panels underwent static immersion from November 30th, 2006 to June 5th, 2007 at the Florida Institute of Technology's (FIT) test site in the Indian River Lagoon, FL USA. All panels were immersed 1 m below the water surface inside galvanized mesh (1.27 cm) cages for protection from predation. Four replicate panels were used for each coating type. After 187 days of exposure, all panels were visually assessed for the predominate types of fouling attached to their surfaces (e.g., barnacles, tubeworms, oysters) and barnacle adhesion measurements were made in shear (i.e., MPa) following the guidelines set forth in ASTM D3623-78a and AsTm D5618-94, respectively. (22), (25)

University of Hawaii

The siloxane--urethane coatings and IS 425 coated on 4" x 8" aluminum panels underwent static immersion from March 26, 2007 to April 26, 2008 at the University of Hawaii's (UH) Ford Island test site in Pearl Harbor. Honolulu, Hawaii, USA. Four replicate panels of each coating were supported from racks mounted on pilings beneath the testing pier and were immersed I m below the water surface (non-caged). After 395 days of exposure, all panels were visually assessed for the predominate types of fouling attached to their surfaces (e.g., barnacles, tubeworms, oysters) and barnacle adhesion measurements were made in shear (i.e.. MPa) following the guidelines set forth in ASTM D3623-78a and ASTM D5618-94, respectively. (22), (25)

Statistical analysis

For laboratory evaluations, statistical analysis was performed using JMP 6.0.0, SAS Institute Inc. One-way ANOVA's were used to evaluate the differences in reattached barnacle adhesion on the experimental siloxane--polyure thane and control coatings using the original and improved reattachment methods. The p values were reported and Tukey-LKramer HSD was used to compare individual coatings within the datasets ([alpha] = 0.05).

Statistical analysis of ocean immersion barnacle adhesion was carried out at FIT using a two-way ANOVA. Statistical differences among the coatings were determined using the Holm-Sidak method (i.e., All Pairwise Multiple Comparison Procedures) ([alpha] = 0.05).

For ocean immersion barnacle adhesion carried out at UH, the Shapiro--Wilks W test was used on the data as a test of goodness of fit to a normal distribution. Proportional data (e.g., percent cover) were arcsine transformed and other data (e.g., force data) were log transformed to meet these criteria, lithe criteria were met, comparisons of multiple groups were conducted by ANOVA. If the data only violated the assumption of equal variances, comparisons of multiple groups were performed by the Welch test. If the data violated both assumptions, a Kruskal--Wallis rank sum test was used for comparisons between groups. Wilcoxon rank sum tests were employed for multiple pair wise comparisons with a correction for multiple tests ([alpha] = 0.05).

Results

Original vs improved laboratory reattachment method

The evaluation of adult barnacle adhesion to the experimental siloxane--polyurethane and control coatings using the original and improved laboratory reattachment method is shown in Fig. 6. Significant differences in barnacle adhesion among the coatings were observed for both reattachment methods (p < 0.0001).

[FIGURE 6 OMITTED]

With respect to the original reattachment method (black bars), only the polyurethane control coating (POLY) was determined to be significantly different among the coatings using the Tukey--Kramer comparison test. In this regard, the POLY coating exhibited a substantially higher degree of barnacle adhesion (0.25 MPa) than the rest of the coatings evaluated (0.05-0.10 MPa) (Table 2). With respect to the improved reattachment method (white bars), several of the coating compositions exhibited a statistically significant difference in barnacle adhesion. As observed for the original reattachment method, the POLY control coating also exhibited a statistically higher degree of barnacle adhesion (0.46 MPa) than the rest of the coatings evaluated (0.05-0.29 MPa). The T2 coating exhibited statistically similar barnacle adhesion values to both siloxane--polyurethane coatings containing the poly-caprolactone end groups attached to the poly(dimethylsiloxanc) (PDMS) backbone (PU-PCL-3 and PU-PCL-4), but was significantly higher than the aminopropyl-terminated PDMS containing coatings (PU-3 and PU-4). The PU-3 and PU-4 coatings were shown to have statistically similar barnacle adhesion values, with the PV-3 coating showing significantly lower barnacle adhesion than PU-PCL-3 and PU-PCL-4. Barnacle adhesion to PU-4 was shown to be significantly lower than PU-PCL-3, but statistically similar to PU-PCL-4. The International Paint fouling-release coating systems (IS 425 and IS 900) exhibited significantly lower barnacle adhesion values (0.05 and 0.06) than the rest of the coatings evaluated (0.120.46 MPa), but were shown not to be statistically different from each other.
Table 2: Evaluation of experimental siloxane-polyurethane and
control coatings with original and improved laboratory
reattachment methods

Coating        Reattached  Barnacles not    Barnacles    Barnacle
           adhesion (mean       attached   moved from    breakage
          [+ or -] stdev)            (#)     original      during
                                          positioning     removal
                                                  (#)         (#)
                   NIT/IT         NIT/IT       NIT/IT      NIT/IT

PU-3        0.05 [+ or -]            1/0          1/0         0/0
          0.05/0.12 [+ or
                  -] 0.05

PU-4        0,09 [+ or -]            0/0          1/0         0/0
          0.05/0.14 [+ or
                  -] 0.01

PU-PCL-3    0.10 [+ or -]            0/0          2/0         0/0
          0.07/0.24 [+ or
                  -] 0.06

PU-PCL-4    0.08 [+ or -]            0/0          0/0         0/0
          0.06/0.22 [+ or
                  -] 0.04

T2          0.09 [+ or -]            3/0          4/0         1/1
          0.08/0.29 [+ or
                  -] 0.04

POLY        0.25 [+ or -]            0/0          2/0         2/6
          0.09/0.46 [+ or
                  -] 0.16

IS 425      0.05 [+ or -]            1/0          1/0         0/0
          0.02/0.06 [+ or
                  -] 0.03

IS 900      0.05 [+ or -]            0/0          6/0         0/0
          0.03/0.04 [+ or
                  -] 0.01

NIT no immobilization template, IT immobilization template,
barnacle breakage visible damage to shell or base plate, stdev
standard deviation of the mean


In addition to adhesion measurements, the total number of barnacles that did not reattach and/or moved from their original positioning during the 14 days of immersion in ASW was also determined for both the original and improved reattachment methods (Table 2). As one might expect, no unattached or relocated barnacles were observed for any of the coatings evaluated with the improved reattachment method that employed the use of the immobilization templates. However, several barnacles did not reattach and/or moved from their original positioning using the original reattachment method. In this regard, the PU-3 and IS 425 coatings each had one barnacle that did not reattach to the coating surface and had also moved from its original positioning. Three barnacles were also determined to have not reattached and had moved from their original positioning on the T2 coating surface. Of the eight coatings evaluated, only the PU-PCL-4 coating retained all nine barnacles in their original positioning on the coating surface. In contrast, the T2 and IS 900 coatings had four and six relocated barnacles, respectively. The remaining five coatings had one or two barnacles that moved during the reattachment period.

Figure 7 illustrates the relocation of barnacles on the IS 900 and IS 425 coating surfaces after the 14 days of immersion in ASW using the original reattachment method. In several instances, small air bubbles were observed underneath the barnacles shortly after the coatings had been immersed in the ASW aquarium tanks. These air bubbles appeared to facilitate the movement of barnacles across the coating surfaces as they repeated the process of extending and retracting their cirri (Fig. 7).

[FIGURE 7 OMITTED]

The occurrence of barnacle shell breakage during force gauge measurements was also recorded for both reattachment methods. In this regard, the T2 and POLY control coatings exhibited significant shell breakage during the force gauge measurements using both the original and improved reattachment method. However, a larger number of barnacles had a fractured or damaged shell on the POLY control coating using the improved reattachment method (6 of 9 barnacles) vs the original reattachment method (2 of 9). Figure 8 provides an image of the shell fracture/breakage that was observed for barnacles reattached to the POLY control coating. In several instances, complete cohesive failure between the barnacle shell and base plate was observed where the entire base plate remained firmly adhered to the surface of the POLY control coating.

[FIGURE 8 OMITTED]

Effect of barnacle basal diameter

A comparison of reattached barnacle adhesion on the IS 425, IS 900, T2, and POLY control coatings using the improved reattachment methodology with barnacle base plate diameters of 5-6 mm and 7-8 mm is shown in Fig. 9. The adhesion was significantly higher for the 5-6 mm diameter barnacles on the T2 (p <0.0002) and IS 425 (p <0.0238) coatings. In addition, it is clear that adhesion to the POLY coating was also substantially higher for the 5-6 mm diameter barnacles. In this regard, all nine reattached barnacles exhibited base plate damage or shell fracture during force gauge measurements for the 5-6 mm diameter barnacles while only six of the nine reattached barnacles exhibited this phenomenon for the 7-8 mm diameter barnacles. Furthermore, the adhesion was slightly higher for the 5-6 mm diameter barnacles (0.12 MPa) than the 7-8 mm diameter barnacles (0.10 MPa) on the IS 900 coating, but the difference was not statistically significant (p < 0.0600). It is also worth noting that three of the nine 5-6 mm diameter barnacles also exhibited base plate damage or shell fracture on the T2 coating, similar to the POLY coating, which was not observed for the 7-8 mm diameter barnacles.

[FIGURE 10 OMITTED]
Coating           Barnacle adhesion (MPa)

T2      5-6mm                         6/3
        7-8mm                         9/0

POLY    5-6mm                           *
        7-8mm                         3/6

IS 425  5-6mm                         9/0
        7-8mm                         9/0

IS 900  5-6mm                         9/0
        7-8mm                         9/0

Note: Table made from bar graph.

Fig. 9: Evaluation of barnacle adhesion to the IS 425, IS 900, T2, and
POLY control coatings using the improved reattachment methodology with
barnacle base plate diameters of 5-6 mm (black bars) and 7-8 mm (white
bars). The asterisk indicates that all nine reattached barnacles
exhibited significant base plate damage or shell fracture during force
gauge measurements. Ratio above each bar = number of barnacles with
measurable removal force/number of barnacles that exhibited
significant base plate damage or shell fracture during force gauge
measurements. Error bars represent one standard deviation of the mean
barnacle adhesion values


Static ocean immersion testing

Florida Institute of Technology

The results of static ocean immersion testing after 187 days of exposure at the FIT field testing site (Indian River Lagoon) are provided in Fig. 10. All of the experimental and control coatings examined were heavily fouled with barnacles, but also included a variety of arborescent and encrusting bryozoans, oysters, sponge and calcareous and soft tubeworms. Only coatings PU-3. PU-4, and the IS 425 control coating provided a measurable barnacle adhesion value (T2 and IS 900 control coatings were not included in field evaluations). Barnacles attached to the PU-PCL-3, PU-PCL-4, and POLY control coatings could either not be removed at the maximum reading of the force gauge (10 lb) or exhibited significant shell breakage when dislodged from the coating surface. The IS 425 control coating was shown to have a significantly lower average adhesion value (0.10 MPa) than t he PU-3 and PU-4 coatings (0.14-, p < 0.0019 and 0.13 MPa: p < 0.0042). It is important to note that the IS 425 control coating showed higher adhesion values than typically observed for this coating system (0.040.07 MPa) at the FIT field testing site (personal communication with Prof. Geoff Swain).

University of Hawaii

Figure 11 shows the results of static ocean immersion testing after 395 days of exposure at the UH lield testing site (Ford Island). With regards to the accumulation of fouling, barnacles and tubeworms were the most prevalent hard fouling organisms while hydroids, filamentous green algae, and colonial tunicates were the most common soft organisms on both the experimental and control coatings. The PU-3 and PU-4 coatings exhibited barnacle adhesion values (0.21 and 0.23 MPa) that were statistically similar (p < 0.0001) to those obtained on the IS 425 fouling-release control (0.18 Mpa). In contrast, the PL5-PCL-3 and PU-PCL-4 coatings exhibited barnacle adhesion values (0.35 and 0.43 MPa) that were significantly p < 0.0001) higher than both the IS 425 control and the PU-3 and PU-4 coatings.

[FIGURE 11 OMITTED]

Discussion

The original reattachment methodology developed by the authors has been utilized to screen the fouling-release performance of marine coatings generated with both conventional methods (24), (26) and a combinatorial, high-throughput approach. (13), (14) (16), (17) Although this laboratory technique has been successfully utilized to screen the fouling-release performance of new marine coating technologies, a number of improvements have been made to streamline and enhance the overall utility or the reattachment methodology.

One of the most significant improvements to the barnacle reattachment methodology was the development of a template to immobilize barnacles on the coating surfaces during the entire reattachment process. The implementation of this immobilization template has dramatically improved the discriminatory capability of the reattachment methodology. As shown in Fig. 6, significant differences in barnacle adhesion were clearly observed for several coating compositions when the immobilization templates were utilized. With respect to the experimental siloxane--polyurethane coatings, the aminopropyl-terminated PDMS component was more effective at mitigating barnacle adhesion than its poly-caprolactone counterpart, where the adhesion strength of barnacles reattached to the aminopropyl-terminated PDMS containing coatings was approximately half of what was observed for the poly-caprolactone coating compositions. The original reattachment method, however, was unable to show a discernable difference in barnacle adhesion values based on these two compositional components. Furthermore, and in contrast to the improved reattachment method, the original methodology was unable to detect significant differences in barnacle adhesion between the experimental siloxane--polyurethane coatings and the commercial fouling-release coating systems from International Paint. The only agreement observed between the original and improved reattachment methodologies was the barnacle adhesion values obtained on the POLY control coating, which was significantly higher than the rest of the coatings evaluated is this study.

It is also important to point out that the reattached barnacle adhesion values were considerably higher when the immobilization templates were employed (Table 2). This is especially true for the experimental siloxane--polyurethane, T2, and POLY control coatings where twice the adhesion strength, on average, was obtained using the improved reattachment method. In addition, three times as many barnacles exhibited a significant degree of shell breakage or base plate fracture during force gauge measurements on the POLY control coating when the immobilization template was utilized. This indicates that cohesive failure, between the barnacle shell and the adhesive plaque, occurred at a higher frequency than adhesive failure, between the adhesive plaque and the surface of the PU control coating, using the improved reattachment method. As indicated by Wendt and colleagues, this phenomenon is typically observed on non-easy release surfaces as the adhesion between the coating surface and the barnacle adhesive plaque is so great that the base plate cannot withstand the forces required to remove or dislodge the barnacle. (19)

It is also important to note that the authors of the present study previously reported that the rate of adhesion was to some degree coating dependent, with the rate of adhesion influenced by the specific surface properties of the coating(s) undergoing barnacle reattachment. (15) In particular, it was shown that a relatively hydrophilic polyurethane coating (the same PU formulation used in this study) required at least 4 weeks of reattachment to achieve an average adhesion value substantially higher than the hydrophobic silicone elastomer coating, Silastic T-2. However, the results of this study indicate that the improvements made to the reattachment methodology, even when excluding the use of an immobilization template, increased the rate of reattached barnacle adhesion to the PU control coating, as evidenced by the 0.25 MPa average adhesion value obtained after only 2 weeks of reattachment (0.28 MPa average adhesion after 4 weeks in previous study by the authors). When the immobilization template was utilized, however, the average adhesion value was almost twice as high (0.46 MPa average adhesion value) after the same duration of reattachment (2 weeks). In contrast, the adhesion of reattached barnacles to the International Paint coatings, IS 425 and IS 900, was not significantly higher when the immobilization template was utilized. These results are consistent with our previous study showing that the IS 425 coating achieved maximum adhesion strength after 1 week of reattachment with no significant change in adhesion strength after an additional 3 weeks of reattachment. (15)

Several possible factors may be responsible for improving or enhancing the rate and strength of adhesion using the improved reattachment methodology. First and foremost, the utilization of an immobilization template enables optimal contact to be achieved between the coating substratum and the base plate of the barnacles throughout the entire reattachment process. This not only facilitates optimal contact, but prevents the animals from moving across the coating surfaces when extending and retracting their cirri during the daily feeding regimens of brine shrimp nauplii (Fig. 7). Furthermore, it was suggested by the authors in the previous study that the low rate and tenacity of adhesion observed on relatively hydrophilic coatings (PU control coating) was most likely governed to some degree by the ability of the barnacle cement to exclude water during the adhesion process. When using the immobilization template, the downward force applied by the mesh fabric would be expected to enhance the ability of the barnacle cement to exclude water during the adhesion process, thus increasing the rate and tenacity of adhesion after 2 weeks of reattachment.

In addition to the use of the immobilization template, the selection and utilization of only those barnacles exhibiting good quality base plates ("flat" with no visible damage or "cupping" as shown in Fig. 1) would also be expected to enhance the rate and strength of adhesion. It is also likely that the continuous monitoring and maintenance of water quality in the aquarium tank system would ensure that optimal conditions (pH, salinity, and temperature) are maintained, as opposed to traditional aquarium tanks and other types of water reservoirs that are not monitored and adjusted on a continual basis.

The selection of barnacles with smaller base plate diameters (5-6 mm vs 7-8 mm) for reattachment can also significantly improve the strength or tenacity of adhesion. This increase in adhesion strength for the smaller barnacles was clearly demonstrated on the T2 and PU control coatings (Fig. 9). In this regard, the average adhesion strength of the 5-6 mm diameter barnacles (0.29 MPa) was approximately twice as high as the average adhesion strength for the 7-8 mm diameter barnacles (0.17 MPa) on the T2 control coating surface. Furthermore, three of the nine 5-6 mm diameter reattached barnacles exhibited significant shell breakage during the force gauge measurements on the T2 coating. In contrast, none of the 7-8 mm diameter reattached barnacles exhibited visible shell breakage or base plate damage during force gauge measurements on this coating surface. A similar phenomenon was observed on the PU control coating. All nine of the 5-6 mm reattached barnacles exhibited significant shell breakage or base plate damage during the force gauge measurements, as compared to only six of the nine 7-8 mm diameter reattached barnacles on this coating surface. Based on these results, it is entirely possible that the use of 5-8 mm diameter barnacles for the evaluation of the experimental siloxane--polyurethane and T2, PU, IS 425, and IS 900 control coatings resulted in lower adhesion strengths, and presumably a lower incidence of shell breakage or base plate damage, than would have been observed if only 5-6 mm diameter barnacles were utilized.

The authors hypothesize that the use of 5-6 mm diameter barnacles results in higher adhesion strengths and a higher degree of precision for the reattachment assay, when compared to the 7-8 mm diameter barnacles, because the smaller barnacles are growing and producing their adhesive at a faster rate than the larger barnacles. In this regard, the rate of growth for A. ainphitrite is relatively rapid until they reach a base plate diameter of 5-6 mm. (19) The rate of growth then gradually declines as the barnacles switch from using energy for growth to energy for reproductive processes (most barnacles have mature ovaries by the time their base plate diameter reaches 7-8 aim). As growth rate slows, calcification continues, resulting in a more robust body structure that is capable of withstanding higher shear forces. Berglin and colleagues observed that smaller diameter A. (= Balanus) improvisus were more fragile than larger ones and that failure within the barnacle structure obscured differences in the strength of adhesion to surfaces. (27) However, with the reattachment method using A. amphitrite, failure occurs exclusively between the barnacle adhesive and the surface on very good fouling-release coatings (IS 425 and IS 900) irrespective of the size of the barnacle (Fig. 9). On intermediate fouling-release coatings (T2), a relatively low proportion of reattached A. amphitrite exhibited failure within the body structure when the barnacles were small, as opposed to exclusive failure between the coating surface and the barnacle adhesive when the barnacles were larger and possessed a more robust body structure. On poor fouling-release surfaces (PU), whether barnacles were grown from cyprids on the surface (19), (27-31) or reattached to the surface, (15), (17-26) moderate to high proportions of barnacles broke and fragments of the base plate typically remained on the surface (Fig. 8) for both small and larger base plate diameter barnacles. Considering these observations, the reattachment assay using 5-6 mm diameter barnacles of A. anzphitrite can effectively leverage the incidence of barnacle shell breakage or base plate damage, similar to Wendt et al., (19) as an additional indicator (to mean adhesion strength) of a coatings fouling-release properties (increasing incidence of shell breakage or base plate damage observed as fouling-release properties of a coating(s) decreases).

In addition to improving the rate and strength of adhesion, the improved reattachment methodology is also much more amenable to the process of developing coatings using a high-throughput, combinatorial approach. One of the primary benefits is the ability to maintain proper positioning and orientation of the barnacles on the coating surfaces throughout the entire duration of reattachment. This is critically important as correct positioning and orientation (lateral plates perpendicular to force gauge application) of the reattached barnacles is essential for proper force gauge measurements to be obtained using the semi-automated push-off device (Fig. 4). Translocation of barnacles from the original positioning often occurred with the original reattachment methodology, resulting in barnacles that were either non-attached or attached to a location on the coating surface that was difficult to properly access with the mounted force gauge of the semi-automated push-off device. Furthermore, some barnacles would translocate and reattach to defects in the coating or on the edge of the coating patches where coating film thicknesses are generally lower than in the center of the coating patches. It has been previously demonstrated that coating film thickness can influence the strength of barnacle adhesion to elastomeric coatings where adhesion decreases as the film thickness increases. (19) Thus, coatings that rely predominantly on their bulk modulus properties to facilitate easy release of barnacles would not be expected to perform as well for barnacles reattached to areas of thin film thickness (edge of the coating patch) as they would to thicker areas of the coating patch (center of coating patches). In addition, the occurrence of non-attached barnacles is all hut eliminated using the immobilization template, enabling the maximum number of barnacle adhesion measurements to be obtained for each coating = 9) and increasing the likelihood that statistical differences in barnacle adhesion can be ascertained among the large sets of combinatorial coating arrays evaluated at one time. The utilization of a semi-automated push-off device to dislodge barnacles improves the accuracy and precision of the force gauge measurements by precisely controlling the angle and rate of force gauge application. The utilization of this device ensures a higher probability that consistent and reproducible barnacle adhesion measurements are obtained across large sets of coatings arrays, when compared to the original methodology which relies on manual application of the force gauge by the operator(s). These aforementioned enhancements likely contributed to the smaller variance in the adhesion force measurements (standard deviations) obtained for the improved methodology on the moderate to good fouling-release coatings. Finally, the amount of time required to measure barnacle base plate areas for normalization of calculated adhesion values (MPa) is dramatically reduced using a scanner and the Sigma Scan Pro 5.0 software package, when compared to the tedious and time consuming process of using calipers to measure base plate dimensions (three measurements per barnacle) with the original reattachment methodology.

The primary goal of most laboratory biofouling assays is to obtain a first approximation of a coatings antifouling properties without expending a substantial amount of time, effort, and resources to do so. (12) In most instances, these assays arc aimed at the evaluation of new coating or paint technologies to narrow in on or identify compositions or key compositional components that are effective at mitigating biofouling, typically towards a select or targeted group of marine fouling organisms (e.g., bacteria, algae, barnacles, and tube-worms). Although these assays are often quite effective for discriminating antifouling performance in the laboratory, it is important to seek to understand or establish a relationship between the data generated with these assays and the results garnered with real-world performance evaluations, such as static ocean immersion testing on large raft panels. (11), (15), (32) In this regard, the authors have previously established a good correlation between barnacle adhesion data obtained with the original barnacle reattachment method and barnacle adhesion data from static ocean immersion testing at the FIT field testing site on a series of polysiloxane fouling-release coatings. (15) Likewise, good agreement was also observed in this study between the improved barnacle reattachment methodology and the results of static ocean immersion testing at both the FIT and UH field testing site (Fig. 10, 11). Both the laboratory and field assessments identified the International Paint 425 external control as the best performing coating with respect to barnacle adhesion. Similarly, the aminopropyl-terminated PDMS containing coatings (PU-3 and PU-4) were also shown to have good barnacle release properties in both the laboratory and the field. The poly-caprolactone end group functionalized PDMS coatings (PU-PCL-3 and PU-PCL-4) showed moderate fouling-release performance in the laboratory and poor (UH) to no (FIT) fouling-release performance in the field (in the latter case, barnacles either broke or could not be removed with a hand-held force gauge).

It is possible that the long duration of ocean immersion before evaluation of barnacle adhesion in the field 87 days) resulted in poorer performance (no removal, shell/base plate damage, or high removal forces) than the relatively short term testing conducted in the laboratory (28 days of total water immersion; 2 weeks preconditioning and 2 weeks of reattachment) for the siloxane--polyurethane coatings PU-PCL-3 and PU-PCL-4. The authors have recently reported on a comprehensive biofouling laboratory evaluation of a combinatorial library of siloxane--polyurethane coatings analyzed after 19 days of water imrnersion.16 Additional water immersion of these coating (63 days total) also resulted in substantially higher adhesion values (0.10-0.56 MPa after 63 days vs 0.03-0.30 MPa after 19 days) for reattached barnacles (data not reported). It is also possible that a higher degree of adhesion and incidence of shell breakage/base plate damage would have been obtained in the laboratory, similar to that observed in the field at FIT, if only 5-6 mm diameter barnacles were used for the reattachment evaluation, as opposed to the 5-8 mm diameter barnacles used in this study.

It is also important to point out that the degree or tenacity of barnacle adhesion was substantially different between the two static ocean immersion field testing sites and appeared to be somewhat dependent on the coating composition. In this regard, the IS 425 coating exhibited a much higher adhesion value at UH (0.18 MPa) as compared to that obtained at the FIT field testing site (0.10 MPa). Similarly, the siloxane--polyurethane coatings PU-3 and PU-4 also exhibited a higher degree of barnacle adhesion at the UH testing site (0.21-0.23 MPa) as compared to that observed at the FIT testing site (0.12-0.14 MPa). Interestingly, the siloxane--polyurethane coatings PU-PCL-3 and PU-PCL-4 exhibited a high degree of adhesion at the FIT testing site in which none of the barnacles could be removed without exhibiting significant shell breakage and/or base plate damage. In contrast, these two coating surfaces were able to effectively release barnacles intact at the UH testing site as the attached barnacles exhibited a relatively moderate degree of adhesion (0.35-0.43 MPa). These results may indicate that the siloxane--polyurethane coatings underwent a differential aging process at the two field testing sites and that these differences appeared to be dependent on the general coating composition. This difference in aging could be attributed to a number of factors including variations in the pH, temperature, salinity, and general fouling conditions encountered at the two testing sites during the duration of exposure of the coatings. Furthermore, the coatings evaluated at the UH testing site were immersed almost twice as long (395 days) as those immersed at the FIT testing site (187 days). As discussed above, the duration of aging has been shown to affect coating performance in the laboratory reattachment assay and could also be a contributing factor in the field testing evaluations. It is also possible that there was a difference in fouling-release performance based on the species of barnacle used to assess the coatings. The FIT testing site made adhesion measurements with A. eburneus while the UH testing site carried out adhesion measurements with A. amphitrite. Nevertheless, both field testing sites and the laboratory reattachment assay clearly demonstrated that the siloxane--polyurethane coatings based on the aminopropyl-terminated PDMS were considerably more effective at mitigating barnacle adhesion as compared to their poly-caprolactone counterparts and exhibited performance similar to the IS 425 commercial fouling-release coating system.

Conclusion

The improved barnacle laboratory reattachment method, described here, has been shown to be an effective tool for screening or assessing the fouling-release properties of coatings developed for underwater marine applications. The utilization of a novel immobilization template serves to anchor barnacles in place during the underwater reattachment process, greatly enhancing the rate and tenacity of adhesion, while minimizing the occurrence of non-attached barnacles. Automation of force gauge and barnacle base plate measurements not only dramatically reduces analysis time, but also improves the robustness and reproducibility of the reattachment technique. When executed properly, the improved reattachment methodology can adequately discern differences in coating performance after just 2 weeks of underwater reattachment and correlates remarkably well with results obtained from static ocean immersion testing. As a result, the improvements made to the original reattachment methodology have greatly enhanced the utility of this technique to screen large numbers of coatings in parallel to disqualify poor performing compositions and identify or select only those that warrant further characterization using more traditional and rigorous methods of analysis, such as static ocean immersion testing in the field. When partnered together, the improved laboratory reattachment method and ocean immersion testing can serve as an efficient and cost-effective approach for evaluating the efficacy of new fouling-release coating technologies for use in underwater marine environments.

Acknowledgments The authors would like to thank Prof. Geoff Swain and Emily Ralston, Florida Institute of Technology, and Prof. Michael Hadfield, University of Hawaii, for conducting static ocean immersion testing. Financial support from the Office of Naval Research through ONR Grants N00014-07-1-1099 and N00014-08-1-1149 is gratefully acknowledged.

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J. Coat. Technol. Res., 9 (6) 651-665, 2012

[c] ACA and OCCA 2012

S. Stafslien, J. Daniels, J. Bahr, B. Chisholm, D. Webster

Center for Nanoscale Science and Engineering, North

Dakota State University, 1805 NDSU Research Park Drive, Fargo, ND 58102, USA

e-mail: Shane. Stafslien@ndsu.edu

B. Chisholm, A. Ekin, D. Webster

Coatings and Polymeric Materials, North Dakota State University, Fargo, ND, USA

B. Orihuela. D. Rittschof

Duke University Marine Laboratory, Nicholas School, Beaufort, NC, USA

DOI 10.1007/s11998-012-9409-7
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Author:Stafslien, Shane; Daniels, Justin; Bahr, James; Chisholm, Bret; Ekin, Abdullah; Webster, Dean; Orihn
Publication:JCT Research
Date:Nov 1, 2012
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