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Synthesis, formulation, and characterization of siloxane-polyurethane coatings for underwater marine applications using combinatorial high-throughput experimentation.

Abstract Crosslinked siloxane-polyurethane coatings were designed, synthesized, formulated, applied, and characterized using combinatorial high-throughput experimentation and eight coatings were selected as candidates for further characterization. First, 72 novel hydroxyalkyl carbamate and dihydroxyalkyl carbamate-terminated poly(dimethylsiloxane) (PDMS) oligomers and their carbamate-linked block copolymers with poly([epsilon]-caprolactone) (PCL) were synthesized using a high-throughput synthesis system. These PDMS oligomers and block copolymers were characterized for their molecular weight using high-throughput Gel Permeation Chromatography (Rapid-GPC). The 72 oligomers were then incorporated into siloxane-polyurethane formulations at four different levels resulting in 288 coatings. After initial screening of these 288 coatings, eight coatings were selected for further characterization. Differential scanning calorimetry, dynamic mechanical analysis, X-ray photoelectron spectroscopy and surface energy analysis demonstrate the presence of PDMS on the surface with a polyurethane underlayer. Pseudo-barnacle adhesion and the attachment strength of reattached live barnacles (Balanus amphitrite) were in good agreement. Out of the eight coatings that were down-selected, two coatings performed well in algal (Ulva), bacterial (Cytophaga lytica, Halomonas pacifica), and barnacle (Balanus amphitrite) laboratory screening assays and are potential candidates for ocean testing.

Keywords Surface analysis, Biofouling, Isocyanates, Polyurethanes, Silicones, Marine, Combinatorial, High-throughput experimentation

Introduction

Biological fouling of ship hulls by marine organisms has been a problem for mankind since man built the first ship. Fouling increases drag which decreases ship speed, maneuverability, and increases fuel consumption. In addition, fouling may result in the transportation of marine organisms across ecosystems, which can have severely adverse effects on local marine-based economies. The best solution for combating fouling to date has been the use of toxic paints; however, in some cases this can result in the destruction of non-targeted sea life. (1-3) Fouling-release coatings are a leading nontoxic alternative coating technology to biocide-containing antifouling paints. Fouling-release coatings do not necessarily prevent the settlement of marine organisms but permit their easy removal with application of shear to the surface. (4-6) Coatings with low modulus and low surface energy have been shown to provide easy removal of marine organisms from the coatings' surface. (7)

Poly(dimethylsiloxane)s (PDMS) have very unique properties and have received a lot of attention in marine coating applications due to their low surface energy and low modulus. Pure PDMSs have weak mechanical properties and they cannot be used directly as a polymer for marine coating applications. (8-10) Filled and crosslinked siloxane coatings have shown some potential as fouling-release coatings, (11) however, these have poor adhesion to most substrates and are easily damaged. Polyurethane coatings can provide toughness and have good adhesion to many substrates. Previous work on siloxane-polyurethane block copolymers demonstrated good initial properties, however, their performance under water deteriorated rapidly, due to conversion of the initially hydrophobic polymer into a hydrophilic polymer after water immersion. (12-14)

In order to provide a tough coating system that maintains its properties under water immersion, a crosslinked siloxane-polyurethane system is being explored in order to have a coating with low surface energy, low modulus, good adhesion, good durability, and stability. (15) PDMS has a low surface energy and a low solubility parameter which results in the incompatibility of PDMS with other polymer systems. (9) Due to its low surface energy, PDMS will predominate on the surface of most systems into which it is incorporated; thus, in the siloxane-polyurethane coating system, a self-stratified structure is expected. (15) However, it is important that the chain ends of the PDMS are firmly anchored in the polyurethane phase, thus the composition of the functional groups on the PDMS is expected to play a critical role in the final morphology and properties of the stratified coatings. Thus, in this work, we are exploring the use of PDMS oligomers terminated with hydroxylalkyl carbamate and dihydroxyalkyl carbamate groups to increase the compatibility between the polyurethane underlayer and PDMS top layer. (16) In addition, poly([epsilon]-caprolactone) (PCL) is known to form compatible blends with several polymers such as polyurethane. Therefore, PCL blocks were added to previously functionalized PDMS oligomers to further increase the compatibility between the polyurethane and PDMS. (17-19)

The coating formulation includes a number of components such as the organofunctional siloxanes, crosslinkers, fillers, catalysts, solvents, and other additives. Within each of these coating ingredients there are also many possible variations. For instance, the type of siloxane, type of block copolymer, molecular weight of the siloxane, length of blocks linked to siloxane, type of functional group, etc. Since it is not yet known which specific combination of these variables will yield a coating having the optimum performance, combinatorial high-throughput experimentation is needed to optimize the coating formulations and obtain the best coating properties possible. With the combinatorial approach, libraries of polymers having systematic variations in composition are prepared, and the coatings properties are screened using high-throughput methods. In this way, a large compositional space can be thoroughly explored in order to find compositions that are particularly suitable for the given application. (20,21)

Therefore, in this study, the effect of hydroxyalkyl carbamate-terminated PDMS molecular weight, presence and length of PCL blocks, and the percent loading of siloxane in the coating formulations on the performance of siloxane-polyurethane thermoset coatings is explored. From the initial screening of 288 coatings, 8 coatings were selected for additional characterization, including their interaction with marine organisms.

A number of laboratory-based assays using pseudo-barnacles (22) and a range of relevant test organisms were employed. Two marine bacterial species, Cytophaga lytica (C. lytica) and Halomonas pacifica (H. pacifica), were used to evaluate the fouling-release performance of the siloxane-polyurethane coatings. Bacteria are one of the first fouling organisms to colonize man-made structures submerged in the sea and typically predominate in a multispecies, microbial biofilm, or "slime" layer. Once established on a ship's hull, microbial slimes have been reported to significantly increase hydrodynamic drag (up to 25%) (23,24) and provide cues for the settlement of other fouling organisms, such as algae, (25) and tubeworms. (26) The algal test organism was Ulva, which is the most common macroalga that fouls ships. Dispersal is mainly through microscopic, motile zoospores which are released in large numbers and form the starting point of the laboratory bioassay. The adhered spores settle and rapidly germinate into sporelings (young plants), which adhere weakly to fouling-release coatings. (27) The ease of removal of sporeling biomass was quantified by application of hydrodynamic forces using a water-jet apparatus (28) modified for use with array panels. (29) Adult barnacles (Balanus amphitrite) were used as a representative shell-fouling organism. After hatching, the larvae go through a number of developmental stages culminating in cypris larvae (cyprids), which have to settle (attach) to a surface in order to complete the lifecycle. Once an appropriate surface is located, cyprids settle and quickly metamorphose into juvenile barnacles. Barnacles produce a protein-rich adhesive that permanently cements them to the surface. As with Ulva, adult barnacles adhere weakly to fouling-release coatings. (30)

Experimental

Reagents

Bis(3-aminopropyl)-tetramethyldisiloxane (BAPTMDS) and octamethylcyclotetrasiloxane ([D.sub.4]) were obtained from Gelest, Inc. Ethylene carbonate (EC), 2,4-pentanedione, ethyl 3-ethoxypropionate (EEP), dibutyltin diacetate (DBTDAc), tin(II) 2-ethylhexanoate, methylene iodide (MI), benzyltrimethylammonium hydroxide (40% solution in methanol), methyl isobutyl ketone (MIBK), and HPLC grade water were purchased from Aldrich. TPCL (Tone Polyol 0305) was received from The Dow Chemical Company. IDT (Tolonate IDT 70B) isocyanate was received from Rhodia. IDT is a triisocyanurate resin of isophorone diisocyanate. Polyurethane grade methyl n-amyl ketone (MAK) was received from Eastman. Glycerine carbonate (GC) and [epsilon]-caprolactone ([epsilon]-CL) were provided by Huntsman and Solvay Chemicals, respectively. THF was received from VWR International. DC 3140 and T2 Silastic were received from Ellsworth Adhesives. DC 3140 and T2 Silastic were solvent reduced with MIBK and called Silicone Rubber A and Silicone Rubber B, respectively. All materials were used as received without further purification. For the bacterial bioassays, all media and buffers were prepared with deionized water (18.2 M[OMEGA] [cm.sup.-1]) generated by a Barnstead/NANO-pure/Diamond water purification system. Artificial seawater (ASW) was prepared by dissolving 38.5 g of sea salts (Sigma-Aldrich, St. Louis, MO) per liter of deionized water. Biofilm growth media (BGM) was prepared by supplementing 1 L ASW with 500 mg peptone (C. lytica) or dextrose (H. pacifica) (Becton Dickinson Labware) and 100 mg yeast extract (EMD Chemicals, Gibbstown, NJ). ASW and BGM were filter-sterilized via vacuum filtration utilizing 0.2 [micro]m VacuCap bottle-top filters (Pall Life Sciences, East Hills, NY). Harleco crystal violet solution (0.3% alcohol solution, PML Microbiologicals, Wilsonville, OR) was utilized to stain biofilms. Sanders[R] Great Salt Lake Artemia Cysts were purchased from Florida Aqua Farms Inc. (Dade City, Florida).

Instrumentation

Symyx Library Studio

Library Studio is used to design polymer synthesis and coating formulation libraries. The software allows for both full factorial and statistical experimental designs and these designs are stored in a common database.

Symyx batch reactor system

The batch reactor system is used to synthesize polymer libraries. The system is comprised of a dual-arm liquid dispensing robot housed in a triple glovebox purged with nitrogen. The robot dispenses liquid reagents according to Library Studio formulations. The three center wells in the synthesis platform can hold arrays of reaction vials and up to 288 simultaneous 1-mL reactions can be run with magnetic stirring and heating up to 120[degrees]C. In this study, 6 x 4 arrays of 8-mL vials were used for the synthesis of PDMS oligomers and block copolymers.

Autodose Powdernium

Powdernium is used to dispense solid and powder reagents for polymer synthesis experiments. The system is housed in the triple glovebox.

Symyx rapid-gel permeation chromatography (Rapid-GPC)

Rapid-GPC is used to determine molecular weight averages and polydispersity of the polymer libraries. High-throughput GPC was performed on a Symyx Rapid-GPC with an evaporative light scattering detector (PL-ELS 1000), equipped with 2 x PLgel Mixed-B columns (10 [micro]m particle size) at 45[degrees]C. Solutions of 1 mg/mL sample in THF were prepared before run: calibration was carried out using polystyrene standards and THF was used as eluent at a flow rate of 2.0 mL/min.

Symyx coating formulation system

Formulation system is used to prepare coating formulations according to Library Studio designs. Dispensing of coating ingredients such as the polymer library, crosslinker, solvent, catalyst, pot-life extender, and other additives is done using disposable pipets, and mixing is accomplished with magnetic stirring.

Symyx coating application system

Coating deposition system is used to deposit coating samples on 4" x 8" substrates in an array format. The 24 elements in the 6 x 4 formulation are applied to two 4" x 8" array panels using a liquid dispensing robot and drawn-down using an adjustable doctor blade.

Symyx coating surface energy

The surface energy system measures and averages contact angles of various liquids in both dynamic and static mode and calculates the surface energy. The system receives two 4" x 8" coating array panels. Water and methylene iodide (MI) are used as test liquids and surface energies are calculated using the Owens-Wendt method. The images of three droplets of each test liquid are taken by a CCD camera and the contact angles are determined using image analysis. Advancing and receding contact angles are measured as the test liquid is being applied and withdrawn from the coatings respectively.

Symyx automated pull-off adhesion

The adhesion system is used to determine the adhesive strength of coating to the substrate and adhesive strength of an epoxy to the coating surface (pseudo-barnacle). The instrument can receive two 4" x 8" coating array panels. For the pseudo-barnacle adhesion test, three aluminum studs per coating sample are glued to the coatings using Loctite Hysol Epoxy 1C-LV, the adhesive is cured, and the maximum force at failure is determined. The average of the three values is reported.

Differential scanning calorimetry (DSC)

DSC Q1000 from TA Instruments with an autosampler was used for glass transition temperature ([T.sub.g]) and melting point ([T.sub.m]) determinations. The samples were subjected to a heat-cool-heat cycle from -160[degrees]C to +200[degrees]C by ramping 10[degrees]C per minute for both heating and cooling. The second heating cycle was used to characterize the samples.

Dynamic mechanical analysis (DMA)

DMA Q800 from TA Instruments was used for storage modulus, tan delta, loss modulus, and glass transition temperature ([T.sub.g]) determinations. [T.sub.g] was determined from the peak of the tan delta curve. The samples were subjected to heating from -140[degrees]C to 250[degrees]C with a heating rate of 2[degrees]C/min.

X-ray photoelectron spectroscopy (XPS)

XPS measurements were run on a modified Physical Electronics Model 555 instrument equipped with a non-chromatic Mg K-alpha X-ray source. The samples were run at 200 eV pass energy for survey spectra and at 25 eV pass energy for the high-resolution spectra (energy resolution ~1.3 eV). The angle between the double pass CMA axis and the sample normal is 60[degrees]. The spatial resolution is ~2 mm.

Coating film thickness

Film thicknesses of the coatings were measured using PosiTest DFT thickness gage on aluminum substrates.

[FIGURE 1 OMITTED]

Gloss

Gloss measurements were done using micro-TRI-gloss glossmeter according to ASTM D523-89.

Polymer synthesis

Synthesis of 3-aminopropyl-terminated PDMS

The synthesis of the 3-aminopropyl-terminated PDMS oligomers was done according to a procedure presented by us previously. (18) Three combinatorial experiments were designed with Library Studio and are seen in Fig. 1. The numbers in parentheses in Fig. 1 are target molecular weight of 3-aminopropyl-terminated PDMS. The designs were designated low molecular weight siloxane library with EC and GC (LMSEG), high molecular weight siloxane library with EC (HMSE), and high molecular weight siloxane library with GC (HMSG). Synthesis of 3-aminopropyl-terminated PDMS oligomers was done as follows (Scheme 1). Benzyltrimethylammonium hydroxide (40% solids in methanol) catalyst was mixed with [D.sub.4] and the methanol was removed under vacuum (13 mm Hg). The catalyst + [D.sub.4] mixture and BAPTMDS were placed into the holders of synthesis robot. After dispensing of the catalyst + [D.sub.4] mixture and BAPTMDS with the liquid handling robot, the reaction temperature was adjusted to 80[degrees]C and held for 10 h with magnetic stirring. After the completion of the reaction, the temperature was set to 170[degrees]C and kept for 1 h to decompose the catalyst.

Synthesis of hydroxyalkyl carbamate and dihydroxyalkyl carbamate-terminated PDMS

Synthesis of hydroxyalkyl carbamate and dihydroxyalkyl carbamate-terminated PDMS oligomers were done following the synthesis of 3-aminopropyl-terminated PDMS (Schemes 2 and 3) according to procedure presented by us previously. (16) The 3-aminopropyl-terminated PDMS oligomers that were synthesized were used as reagents. Stoichiometric amounts of GC and EC were dispensed into the vials using the liquid handling robot and Powdernium, respectively, according to the library design. After dispensing the reagents, the reaction was run at 80[degrees]C for 10 h with magnetic stirring. After 10 h, the array was cooled down to room temperature.

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Synthesis of carbamate-linked PCL-PDMS-PCL triblock and carbamate-linked H-type PDMS-PCL block copolymers

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Synthesis of carbamate-linked PCL-PDMS-PCL triblock copolymers and carbamate-linked H-type PDMS-PCL block copolymers was done according to procedure described by us previously. (16) PCL blocks were added to the hydroxyalkyl carbamate and dihydroxyalkyl carbamate-terminated PDMS oligomers synthesized in the previous step (Schemes 2 and 3). The vials containing carbamate-linked functional PDMS oligomers that were not to be reacted with [epsilon]-CL (Row A) were replaced with empty vials. The remaining vials containing carbamate-linked functional PDMS oligomers from the previous step were used as reagents for reaction with [epsilon]-CL. Stoichiometric amounts of [epsilon]-CL were dispensed into the vials with the liquid handling robot and 1 drop of tin(II)-2-ethyl hexanoate in 10% toluene solution (approximately 0.05% v/v tin(II)-2-ethylhexanoate to total solids) was also added to the vials as a catalyst. Following dispensing of the [epsilon]-CL, the reaction was run at 80[degrees]C for 10 h with magnetic stirring, and then the reaction temperature was increased to 120[degrees]C and held for 10 more hours. After total of 20 h of reaction time, the array was cooled down to room temperature.

Coating formulation

Coating formulations are composed of the siloxane libraries, TPCL, 2,4-pentanedione, DBTDAc, and IDT, as seen in Fig. 2. Stock solutions of 30% siloxane library in EEP and 90% TPCL in MAK were prepared. A 1% solution of the catalyst DBTDAc was prepared in MAK. Coating formulations were prepared by adding 10, 20, 30, and 40% by weight siloxane polymers into the formulations as seen in Fig. 3. The amount of catalyst DBTDAc was 0.075% by solids for all coating formulations. The ratio of isocyanate to hydroxyl was adjusted to 1.1:1.0. To all formulations, 10% by solids 2,4-pentanedione was added as a pot life extender. All dispensing and mixing was done using the automated coating formulation system. Siloxane library, TPCL, and 2,4-pentanedione were added to the vials first and mixed overnight. Then, IDT and DBTDAc were added to the vials and were mixed until sufficient viscosity was achieved for coating application.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Coating application and curing

Coating deposition was done using the automated coating application system. Eighty [micro]L of each coating formulation were applied in array format on 4" x 8" aluminum panels (Q-Panel, 0.6 mm thickness, type A: alloy 3003 H14). After application, the panels were left at room temperature for overnight curing. Then the panels were placed into the oven for complete curing for 45 min at 80[degrees]C.

Water aging

The coatings were aged in a recirculating dionized water bath. Clean water is maintained by using a UV sterilizer, submicrometer filter, and an activated charcoal filter.

Data analysis

Data analysis was done using Spotfire 8.0 analysis software.

Biological screening

Bacterial biofilm retention and retraction

Rapid laboratory screening assays used to evaluate bacterial biofilm retention and retraction on coatings cast in multiwell plates have been previously reported. (31,32) Briefly, coating formulations were deposited into modified 24-well polystyrene plates and left at room temperature for overnight curing. Then the plates were placed into an oven for complete curing for 45 min at 80[degrees]C. Cured coatings were inoculated with a BGM suspension of the marine bacterium C. lytica (~[10.sup.8] cells [mL.sup.-1]) or H. pacifica (~[10.sup.5] cells [mL.sup.-1]) and incubated statically at 28[degrees]C for attachment and biofilm growth. C. lytica plates were incubated for 18 h, while H. pacifica plates were incubated for 48 h. Plates were then rinsed with deionized water to remove planktonic (unattached) cells and loosely attached biofilm. Biofilms retained on the coating surfaces were allowed to dry at room temperature and then stained with crystal violet and measured for total biomass (retention) and percent coverage (retraction). Each value was reported as the mean of three replicate measurements. Biofilm retraction was only observed for C. lytica.

Bacterial biofilm adhesion

An automated water jet apparatus was used to evaluate bacterial biofilm adhesion to coatings cast in multiwell plates. (33) Coatings were prepared in multiwell plates and incubated with bacteria as described in the previous section. Plates were rinsed with deionized water to remove any planktonic or loosely attached biofilm. The first column for each coating was not treated with the water jet and served as the initial amount of retained biofilm. The second column for each coating was treated with the water jet for 10 s at a jet impact pressure of 18 kPa for C. lytica and 172 kPa for H. pacifica. Plates were allowed to dry at ambient conditions and then stained with crystal violet and measured for total biomass. Percent removal was determined for each coating by comparing the difference in total biomass between the non-jetted and jetted columns. Each percent removal value was reported as the mean of four replicate measurements.

Barnacle reattachment assay

Adult barnacles (Balanus amphitrite) were dislodged from PDMS (Silastic-T2) panels in shear and nine barnacles were placed on each coating surface (three barnacles on three replicate array patches) (Fig. 9). Barnacles were allowed to sit on surfaces for approximately 3 h and then transferred to empty 15-gallon aquarium tanks. Artificial Sea Water (ASW) was then slowly added to each tank to avoid displacement of barnacles from coating surfaces. Barnacles were allowed to reattach for 1 week with daily exchange of water and feeding with brine shrimp nauplii. Coatings, with 1-week reattached barnacles, were removed from aquarium tanks and placed on the bench. Each barnacle was measured for adhesion using the protocols described in ASTM D5618-94. Force was applied to a barnacle edge plate at the base parallel to the substratum using a handheld mechanical force gauge (Shimpo DFS-5, Cole-Parmer, Vernon Hills, IL) until the barnacle detached from the surface. The area of the barnacle basal plate was measured from scanned images of barnacle base plates using Sigma Scan[R] Pro5.0 image analysis software package. Adhesion in shear was calculated by dividing the measured force required to remove the barnacle by the basal area. The reattached barnacle adhesion measurement for each coating was reported as the mean value of the total number of barnacles that had a measurable detachment force.

Ulva sporeling growth and release

Ulva zoospores were obtained from fertile plants of Ulva linza as previously described by Callow et al. (34) The coatings were deposited onto aluminum panels that were previously primed with Sherwin-Williams Macropoxy 646 epoxy primer. Array panels were leached in water for a total of 9 weeks prior to the experiment. The panels were equilibrated in artificial seawater for 2 h before the start of the experiment. Six replicate plates were incubated in trays (3 per tray) for 3 h in the dark with 300 mL of a spore suspension containing 1 x [10.sup.6] spores/mL. After rinsing to remove unattached spores, the settled spores were cultured in enriched seawater medium in a re-circulating culture system. After 6 days a green lawn of algal sporelings (young plants) covered the panels, which were exposed to a water jet at a range of impact pressures (32, 54, 93, 132, 171, and 210 kPa) and the percentage removal estimated visually.

Results and discussion

The aim of this study was to investigate the effects of functionality of the siloxane (di-functional vs. tetrafunctional), PDMS molecular weight, addition of PCL blocks to PDMS backbone, PCL block length, and amount of siloxane polymer on the fouling-release properties of crosslinked siloxane-polyurethane coatings. Since there were many variables, combinatorial high-throughput experimentation was used to explore the effects of these variables on the properties of the siloxane-polyurethane coatings, as was presented by us previously. (35) For this reason a workflow was developed as seen in Fig. 4. The workflow started from design of the experiment followed by synthesis of the polymers. After the synthesis experiments, polymers were characterized and incorporated into coating formulations. Then, the coatings were applied to substrates and the coatings were characterized after curing. After the analysis of all the data, interesting candidates were selected for further characterization.

Polymer library synthesis

In previous papers, we described in detail the three steps required to synthesize hydroxyalkyl carbamate-terminated PDMS, dihydroxyalkyl carbamate-terminated PDMS oligomers, and PCL block copolymers (carbamate-linked PCL-PDMS-PCL triblock copolymers and carbamate-linked H-type PDMS-PCL block copolymer) from them. (16,18) The synthesis experiments that were carried out in this study consist of three different steps. The first step, as seen in Scheme 1, consists of synthesis of 3-aminopropyl-terminated PDMS oligomers of varying molecular weight. The second step consists of reacting the 3-aminopropyl-terminated PDMS oligomers with either EC or GC resulting in hydroxyalkyl carbamate or dihydroxyalkyl carbamate-terminated PDMS oligomers, respectively, as seen in the first parts of Schemes 2 and 3. The third step consists of reacting the hydroxyalkyl carbamate and dihydroxyalkyl carbamate-terminated PDMS oligomers with [epsilon]-CL to form carbamate-linked PCL-PDMS-PCL triblock copolymers and carbamate-linked H-type PDMS-PCL block copolymers, as seen in the second parts of Schemes 2 and 3.

[FIGURE 4 OMITTED]

For this study, three synthesis library experiments were designed for a total of 72 PDMS-based polymers and block copolymers as seen in Fig. 1. In each of the libraries, the PDMS molecular weight was increased column-wise. The numbers in parentheses indicate the target molecular weight of the 3-aminopropyl-terminated PDMS. Row A in all three libraries consists of the hydroxyalkyl carbamate or dihydroxyalkyl carbamate-terminated PDMS oligomers which do not have PCL blocks linked to the PDMS backbone. Rows B, C, and D in all three library designs consist of adding 2, 3, and 4 [epsilon]-CL units to each of the hydroxyl end groups of the PDMS. The library in Fig. 1a consists of low molecular weight PDMS reacted with either EC or GC (LMSEG). The molecular weight was varied from 2500 to 7500 g/mol. The libraries in Fig. 1b and 1c consist of higher molecular weight PDMS ranging from 10,000 to 35,000 g/mol. In the library in Fig. 1b PDMS is reacted with EC (HMSE), and in the library in Fig. 1c PDMS is reacted with GC (HMSG).

The polymer libraries were designed in such a way to investigate the effect of PDMS molecular weight and PDMS composition on the fouling-release properties of the crosslinked siloxane-polyurethane coatings. The PDMS composition in these libraries has several different variations. These variations are the number of functional groups linked to the PDMS oligomers as a result of the use of the two different cyclic carbonates to obtain hydroxyalkyl carbamate and dihydroxyalkyl carbamate-terminated PDMS oligomers, and the presence and length of PCL blocks linked to the PDMS chain end. Since each synthesis library contains 24 polymers, the synthesis experiments resulted in the preparation of a total of 72 novel hydroxyalkyl carbamate-terminated PDMS oligomers, dihydroxyalkyl carbamate-terminated PDMS oligomers, carbamate-linked PCL-PDMS-PCL triblock copolymers, and carbamate-linked H-type PDMS-PCL block copolymers. These 72 oligomers and block copolymers are each unique and different from each other. Using the combinatorial high-throughput approach, the effect of all these variables on coating properties can be explored in an efficient amount of time.

After synthesis of the 72 oligomers and block copolymers, they were characterized for molecular weight using Rapid-GPC. Figure 5 shows the Rapid-GPC results of the synthesis libraries. As seen from Fig. 5, the molecular weights of the PDMS oligomers and block copolymers increase both column-wise and row-wise as intended.

Coating formulation and screening

Following characterization of the oligomers and block copolymers, they were incorporated into siloxane-polyurethane coatings. The formulation components can be seen in Fig. 2. Stock solutions of the PDMS libraries, TPCL, and the catalyst DBTDAc were prepared in order to dispense the materials more accurately and easily. The siloxane polymers were incorporated into the siloxane-polyurethane coatings at four different levels as seen in Fig. 3: 10, 20, 30, and 40% by mass (based on solids). Since the percent loading of the PDMS polymers was varied from 10 to 40% with 10% increments, 288 coatings from the 72 siloxane oligomers and block copolymers were formulated and applied to aluminum panels using the automated systems. Therefore, the experiment as a whole is designed to explore the effects of siloxane molecular weight (nine levels), type of siloxane (eight levels), and amount of siloxane (four levels) on the fouling-release properties of the siloxane-polyurethane coatings.

The coatings that were prepared were screened for their surface energy using contact angle measurements and also for pseudo-barnacle adhesion. Contact angle and pseudo-barnacle adhesion tests were done initially and after 30 days of water immersion. The tests were done after 30 days of water immersion to check their stability after immersion in water since the ultimate use of these coatings is for underwater marine applications.

In Fig. 6, a scatter plot of the water contact angle results of all 288 coatings after water immersion is plotted vs. number of [epsilon]-CL per hydroxyl group. The color of the data points gets darker as the amount of siloxane in the siloxane-polyurethane coating formulation increases. The size of the data point increases with the molecular weight of PDMS. The shape of the data points depends on the type of carbonate used (EC vs. GC) resulting in hydroxyalkyl carbamate-terminated PDMS vs. dihydroxyalkyl carbamate-terminated PDMS as well as carbamate-linked PCL-PDMS-PCL triblock copolymers vs. carbamate-linked H-type PDMS-PCL block copolymer. First, it can be seen that most of the coatings are hydrophobic (Water Contact Angle >90[degrees]. The fact that the coatings are hydrophobic after water immersion also indicates that these coatings are stable against rearrangement. Also, it can be seen that the water contact angle increases with increase in PDMS molecular weight. The addition of PCL blocks generally decreases the water contact angle and the amount of siloxane does not have a significant effect on the water contact angle of the coatings.

[FIGURE 5 OMITTED]

For further analysis, trends in key variables with the responses averaged over the other independent variables are plotted. Figure 7 shows the surface energy results as a function of the PDMS molecular weight for the coatings. It can be clearly seen that as the molecular weight of the siloxane is increased, the surface energy of the coatings decreases. The decreasing trend in surface energy is similar for coatings formulated from siloxane with either EC or GC.

The pseudo-barnacle adhesion results as a function of PDMS molecular weight, number of [epsilon]-caprolactones, and alkyl carbamate-linking group can be seen in Fig. 8. A key observation is that the adhesion is lowest for the hydroxyalkyl carbamate-terminated PDMS polymers (not containing PCL blocks) with a molecular weight greater than 15,000 g/mol (upper left). The coatings based on the dihydroxyalkyl carbamate-terminated PDMS polymers have higher pseudobarnacle adhesion, presumably due to the higher crosslink density from the tetrafunctional PDMS. Adding PCL units to the PDMS polymers generally results in an increase in the pseudo-barnacle adhesion.

The data can be plotted in a number of different ways to develop a large number of observations from this initial screening study; however, due to lack of space not all of the possible graphs can be presented. The major observations and the findings of the initial screening are as follows:

* All of the coatings were hydrophobic initially and most of the coatings maintained their hydrophobicity after water immersion;

* Both water and methylene iodide contact angle averages increased with the increase in siloxane molecular weight. This trend was seen both before and after 30 days of water immersion;

* The trends in the data are clearer and they deviate less for the coatings after water immersion than before water immersion;

* Water contact angle increases only very slightly (at most ~5[degrees]) as a function of percent loading of siloxane both before and after water immersion;

* Surface energy decreases as percent of siloxane polymer was increased (from ~21 to ~16 mN/m);

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

* As the percent of siloxane polymer increases, the pseudo-barnacle adhesion also increases;

* There was no significant surface energy difference between the coatings prepared from siloxanes containing EC and GC; and

* PCL block length did not significantly affect the coating properties.

The screening of the 288 coatings generates a significant amount of data that can then be used to identify the important effects of the independent variables on the key properties of the coatings.

Down-selected coatings

After the initial screening of all 288 coatings, 8 coatings were down-selected as candidates for further characterization. The down-selected coatings and their compositions are given in Table 1. Note that all eight coatings were assigned to new array positions as illustrated in Fig. 9 and listed in Table 1.

[FIGURE 8 OMITTED]

Coatings were prepared from these siloxane oligomers with the same components as before (Fig. 2). In the new array position, column 2 consists of siloxane molecular weight of 10,000 g/mol and column 3 consists of siloxane molecular weight of 35,000 g/mol. Column 1 was filled with standard coatings during coating application to compare the coating performance (Fig. 9). Since the lowest surface energy and lowest pseudo-barnacle adhesion were obtained from PDMS polymers having a molecular weight of 35,000 g/mol, it was selected for further characterization. In addition, both the 10,000 g/mol and 2500 g/mol PDMS polymers gave coatings having the lowest pseudo-barnacle adhesion; it was decided to select the 10,000 g/mol polymer over the 2500 g/mol polymer because lower surface energy was obtained for the 10,000 g/mol polymer. Out of the down-selected eight coatings, four were based on siloxane oligomers prepared using EC and four were based on siloxane oligomers prepared using GC. For the set of four coatings prepared using EC, two were hydroxyalkyl carbamate-terminated PDMS oligomers and two were carbamate-linked PCL-PDMS-PCL triblock copolymers. In addition, for the set of four coatings prepared using GC, two were dihydroxyalkyl carbamate-terminated PDMS oligomers and two were carbamate-linked H-type PDMS-PCL block copolymers. Since the PCL block length did not affect the performance of the coatings significantly, three [epsilon]-CLs per hydroxyl were selected as the PCL block length. The amount of siloxane in the coating formulation was chosen to be 20% by weight based on our previous studies.

Both static and dynamic contact angles of the coatings were measured as seen in Table 2. The coatings based on siloxane molecular weight of 35,000 g/mol (A3, B3, C3, and D3) have higher water contact angles than the coatings based on 10,000 g/mol (A2, B2, C2, and D2). Therefore, molecular weight of 35,000 g/mol has lower surface energy than molecular weight of 10,000 g/mol. The reason for the increase in contact angle is believed to be due to the increase in roughness for the coatings containing siloxane oligomers having 35,000 g/mol. In addition, also from Table 2, coatings prepared from siloxanes containing PCL blocks (B2, B3, D2, and D3) have lower contact angles than coatings prepared from siloxanes without PCL blocks (A2, A3, C2, and C3). The reason for the lower contact angles for coatings prepared from siloxanes containing PCL blocks is thought to be due to an increase in the compatibility of siloxane and polyurethane resulting in smoother coatings. The surface roughness of the coatings is reflected in the gloss data shown in Table 3 and is consistent with these observations.

[FIGURE 9 OMITTED]

Table 3 displays the pseudo-barnacle adhesion, film thickness, storage modulus, and gloss values of the down-selected coatings. Lower pseudo-barnacle adhesion values were obtained from the coatings prepared from hydroxyalkyl carbamate-terminated PDMS (A2 and A3) than from dihydroxyalkyl carbamate-terminated PDMS (C2 and C3). Addition of PCL blocks to hydroxyalkyl carbamate-terminated PDMS increases the pseudo-barnacle adhesion (A2 and A3 vs. B2 and B3). On the other hand, addition of PCL blocks to dihydroxyalkyl carbamate-terminated PDMS decreases the pseudo-barnacle adhesion (C2 and C3 vs. D2 and D3). Pseudo-barnacle results show that samples A2, A3, B2, B3, D2, and D3 have better release properties than samples C2 and C3. No trend or correlation between the coating composition and storage modulus of the coatings was observed. A significant decrease in gloss values of the coatings at all angles can be seen easily when molecular weight of the siloxane in the coating formulation is increased from 10,000 to 35,000 g/mol. The results of the gloss measurements follow the same trend as the contact angle results. The change in both contact angle and gloss values when molecular weight of siloxane is increased might be due to change in the roughness of the coatings as discussed above.

Figures 10 and 11 illustrate the phase separation in the coatings that were prepared. A weak PDMS [T.sub.g] at -122[degrees]C and a polyurethane [T.sub.g] at 145[degrees]C are seen in the DSC plot in Fig. 10. A striking observation is the PDMS melting transition at -50[degrees]C. PDMS melting is only seen for pure PDMS and the observation in this coating system signifies good phase separation between PDMS and polyurethane. The DMA plot in Fig. 11 also shows PDMS [T.sub.g] at -120[degrees]C and polyurethane [T.sub.g] at 190[degrees]C. The storage modulus decreases sharply at -50[degrees]C which signifies PDMS melting which is due to phase separation. In addition, the strong tan delta peak at 190[degrees]C shows that the coating is dominated by polyurethane. XPS analysis of the coatings indicates that the surfaces of the coatings are dominated by PDMS (Table 3), confirming the stratified nature of these coatings.

Bacterial biofilm assays using marine bacteria were used as a screening tool to further characterize the coatings. C. lytica and H. pacifica biofilm results are summarized in Table 4. In addition, representative images of C. lytica biofilm retention and retraction can be seen in Fig. 12. In Fig. 12, all coatings were determined to have retained the same amount of C. lytica biomass, however the surface coverage of the biofilm was affected by the composition of the coating. H. pacifica does not demonstrate a measurable degree of biofilm retraction as observed for C. lytica; therefore, only percent removal can be used for determination of coating performance for this bacterium. The results of both C. lytica and H. pacifica show that samples A2 and C3 are the worst perfomers, while samples A3, B2, B3, C2, D2, and D3 perform better or somewhat comparable to Silicone Rubber A in terms of fouling-release properties.

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

The adhesion strength of reattached barnacles can also be seen in Table 4. It can be seen that all experimental coatings performed better than the standard coatings Silicone Rubber A and Silicone Rubber B. Among the siloxane-polyurethane coatings, samples B2 and C2 are the poor performers, and the rest of the samples A2, A3, B3, C3, D2, and D3 are better performers in terms of fouling-release properties. Lower adhesion values were obtained from the coatings prepared from hydroxyalkyl carbamate-terminated PDMS (A2 and A3) than from dihydroxyalkyl carbamate-terminated PDMS (C2 and C3). Addition of PCL blocks to hydroxyalkyl carbamate-terminated PDMS increases the adhesion values of reattached barnacles (A2 and A3 vs. B2 and B3). On the other hand, addition of PCL blocks to dihydroxyalkyl carbamate-terminated PDMS decreases the adhesion values (C2 and C3 vs. D2 and D3). Pseudo-barnacle and reattached Balanus amphitrite adhesion values are parallel and in a very good agreement.

[FIGURE 12 OMITTED]

The fouling-release properties of the coatings were also evaluated using Ulva sporelings, which had been cultured for 6 days on the array panels. Table 5 summarizes the results for Ulva sporeling removal at different surface impact pressures. At low pressures (34 and 54 kPa) Silicone Rubber A and Silicone Rubber B had better release of biomass than any of the experimental samples. Of the experimental coatings, samples A2, A3, B2, and D2 performed best, all having 80% of the biofilm removed at 54 kPa impact pressure. The least good coatings in terms of release of Ulva sporelings were B3, C2, C3, and D3; even at the highest pressure used (210 kPa) only 60% of the biomass was removed.

An interesting observation is the trend of increased release of soft fouling, i.e., bacteria and Ulva, in relation to the molecular weight of the PDMS. For example, D2 (lower molecular weight PDMS) had higher removal of bacterial biofilm and Ulva sporelings than D3 (higher molecular weight PDMS). The opposite trend is seen for the adhesion strengths of the pseudo-barnacles and reattached live barnacles. The data indicate the need to employ a range of test organisms. Subsequent field assays will provide additional information on the performance of the coatings.

Conclusion

Libraries of 72 novel siloxane polymers were synthesized using an automated synthesis system and the polymers were screened in 288 polyurethane coatings at four levels. Increase in siloxane molecular weight increases water and methylene iodide contact angles and decreases surface energy of the coatings. Initial and after 30 days of water immersion tests results show similarities and the trends look better after 30 days of water immersion. Most of the coatings retain their hydrophobicity after 30 days of water immersion. Addition of PCL blocks to PDMS backbone increases pseudo-barnacle adhesion with siloxanes having difunctional groups but decreases pseudo-barnacle adhesion with siloxanes having tetra-functional groups. XPS, DMA, and DSC results demonstrate phase separation as well as self-stratification of PDMS to the coating surface, which was intended in order to provide good release properties. The adhesion strength of reattached barnacles (Balanus amphitrite) and pseudo-barnacle are in a very good agreement. From all the test results, coatings A3 and D2 performed well in all tests and are candidates for ocean testing.

Acknowledgments The authors would like to thank David A. Christianson for his assistance with the use of combinatorial high-throughput instruments and the Office of Naval Research for supporting this research under Grants N00014-04-1-0597 and N00014-05-1-0822. We also thank John H. Thomas, III, from the University of Minnesota IT Characterization Facility for his assistance with XPS measurements and analysis.

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[c] FSCT and OCCA 2007

This paper was awarded Second Place in the 2006 Roon Awards competition, held as part of the FutureCoat! conference, sponsored by the Federation of Societies for Coatings Technology, in New Orleans, LA, on November 1-3, 2006.

A. Ekin, D. C. Webster ([mailing address])

Department of Coatings and Polymeric Materials, North Dakota State University, Fargo, ND 58105, USA e-mail: Dean.Webster@ndsu.edu

J. W. Daniels, S. J. Stafslien

Center for Nanoscale Science and Engineering, North Dakota State University, Fargo, ND 58102, USA

F. Casse, J. A. Callow, M. E. Callow

School of Biosciences, The University of Birmingham, Birmingham B15 2TT, UK
Table 1: Composition of the down-selected coatings

 Target PDMS Number of
New array Array position in [M.sub.n] Type of [epsilon]-CL per
position synthesis library (g/mol) carbonate hydroxyl

A2 A1-HMSE 10,000 EC 0
A3 A6-HMSE 35,000 EC 0
B2 C1-HMSE 10,000 EC 3
B3 C6-HMSE 35,000 EC 3
C2 A1-HMSG 10,000 GC 0
C3 A6-HMSG 35,000 GC 0
D2 C1-HMSG 10,000 GC 3
D3 C6-HMSG 35,000 GC 3

Table 2: Contact angle and surface energy analysis of the down-selected
coatings

New array Static water CA Static MI CA Surface energy
position (deg) (deg) (mN/m)

A2 113.08 86.26 15.29
A3 117.99 93.21 11.94
B2 105.99 87.05 16.69
B3 112.78 99.73 11.05
C2 111.34 96.57 12.26
C3 115.69 89.35 13.68
D2 106.83 90.92 15.20
D3 113.40 86.47 15.15

New array Advancing water Receding water CA hysteresis
position CA (deg) CA (deg) (deg)

A2 113.92 94.38 19.54
A3 122.03 101.24 20.79
B2 112.03 94.58 17.45
B3 119.56 99.77 19.79
C2 115.06 94.25 20.79
C3 109.50 94.02 15.49
D2 110.11 89.32 20.79
D3 117.79 95.74 22.05

Table 3: Pseudo-barnacle, film thickness, storage modulus, XPS, and
gloss results of the down-selected coatings

New array Pseudo-barnacle Film thickness Storage modulus
position adhesion (N) ([micro]m) at 25[degrees]C (MPa)

A2 9.464 40 1069
A3 9.473 52 1100
B2 9.669 53 645
B3 11.615 48 1335
C2 28.206 40 1315
C3 23.761 50 1309
D2 12.845 36 1120
D3 17.321 50 1069

New array % PDMS coverage Gloss
position from XPS 20[degrees] 60[degrees] 85[degrees]

A2 98.89 18.2 50.7 87.0
A3 99.07 9.2 36.7 72.2
B2 97.45 22.8 56.0 83.1
B3 99.30 14.5 45.4 63.9
C2 99.73 28.1 58.1 79.7
C3 99.59 25.5 56.5 67.7
D2 98.94 30.3 63.5 86.5
D3 99.56 17.4 50.5 69.1

Table 4: Cytophaga lytica biofilm retraction and removal, Halomonas
pacifica biofilm removal, and Balanus amphitrite reattached adhesion

New array C. lytica H. pacifica Balanus amphitrite
position % Cover. % Remov. % Cover. % Remov. adhesion (kPa)

A2 100 51 N/A 43 77
A3 66 51 N/A 43 57
B2 53 66 N/A 75 98
B3 53 60 N/A 69 93
C2 100 52 N/A 73 97
C3 100 48 N/A 8 87
D2 28 72 N/A 70 88
D3 60 53 N/A 59 65
Silicone 100 55 N/A 64 123
 Rubber A
Silicone - - N/A - 131
 Rubber B

Table 5: Ulva sporeling removal from test coatings at different impact
pressures

 % removal of biomass at different
 impact pressures (kPa)
New array position 34 54 93 132 171 210

A2 60 80 99 99 99 99
A3 60 80 95 99 99 99
B2 40 80 99 99 99 99
B3 0 5 20 30 40 60
C2 0 5 20 30 40 60
C3 0 0 20 30 40 60
D2 30 80 95 99 99 99
D3 0 5 20 30 40 60
Silicone Rubber A 80 95 99 99 99 99
Silicone Rubber B 80 95 99 99 99 99
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Author:Ekin, Abdullah; Webster, Dean C.; Daniels, Justin W.; Stafslien, Shane J.; Casse, Franck; Callow, Ja
Publication:JCT Research
Date:Dec 1, 2007
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