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Replacement of traditional seawater-soluble pigments by starch and hydrolytic enzymes in polishing antifouling coatings.

Abstract The use of starch and hydrolytic enzymes as replacement for traditional polishing pigments (e.g., [Cu.sub.2]O and ZnO) in antifouling coatings has been investigated. The enzymes facilitate a slow conversion of water-insoluble starch into water-soluble glucose, and dissolution of glucose causes the development of a leached (porous) layer in the wetted, outermost part of the coating. Subsequent water-binder interaction at the pore walls gives rise to polishing, in a manner similar to that of conventional antifouling coatings. Different starch types have been evaluated and classified as potential coating ingredients, and the impact of the addition of starch on the functional properties of the coating is described. Starches from rice, corn, and tapioca have been tested, and due to a smaller amount of water-soluble content and lesser tendency to agglomerate, corn starch is preferred. Leaching occurs in all the starch-enzyme coatings tested; however, polishing is only detected for two out of four binder systems investigated. Suitable polishing rates of 7-10 [micro]m/month, based on the enzymatic starch-degradation, have been measured. Controls containing only starch (no enzyme) did not polish.

Keywords Pigment, Coating, Polishing, Leaching, Starch, Glucose, Enzyme
List of symbols

Ci         Starch type number i originating from corn
[C.sub.s]  Seawater solubility (mol/[m.sup.3])
M          Molar mass (kg/mol)
Ri         Starch type number i originating from rice
T          Starch type originating from tapioca

Greek symbols

[alpha]    Seawater solubility (dimensionless)
[rho]      Density (kg/[m.sup.3])

Abbreviations

CPVC       Critical pigment volume concentration (vol%)
DFT        Dry film thickness ([mu]m)
LLT        Leached layer thickness ([mu]m)
OA         Oil absorption
PSD        Particle size distribution
PVC        Pigment volume concentration (vol%)


Introduction

Biofouling on ship hulls causes increased drag resistance of the vessel. This lowers the ship velocity at constant power input, and as a consequence more fuel is consumed to maintain sailing speed. Therefore, marine biofouling is an economic burden for ship operators. Also, the increase in fuel consumption increases the emission of greenhouse gasses and other combustion gasses of environmental importance such as [SO.sub.2] and [NO.sub.x]. This ultimately makes ship-hull fouling an environmental issue. Furthermore, manoeuvrability of a biofouled ship may be compromised by the increased roughness of the hull, and biofouling may effectively disintegrate the coating layers and thereby induce corrosion of the steel. (1)

Background

Working mechanism of antifouling coatings

Most antifouling coatings work by releasing biocides (seawater-soluble pigments and organic compounds) into seawater. This leaves behind a leached layer of biocide-depleted coating. If the thickness of the leached layer is continuously increasing to prohibitive values, the diffusion resistance of the dissolved biocidal compounds increases, and for most compounds this means that the flux of biocide at the interface between coating and seawater decreases to inefficient levels. This limits the service life of a coating, though it still contains biocide. The leached layer thickness (LLT) should therefore remain constant and low (preferably 5-20 [micro]m), and this is achieved by a steady erosion of the outermost layer of the leached binder system. (1) This mechanism is called polishing, and is an important capacity of modern chemically active antifouling coatings. Figure 1 shows the mechanisms involved in polishing of an antifouling coating, dividing the process into three steps. (1) A freshly immersed antifouling coating will leach seawater-soluble pigments into the sea. (2) Seawater-filled pores left behind by the seawater-soluble pigments constitute the leached layer of the coating. The surface area of the water-binder interface is increased by formation of the leached layer, which allows for more water-binder interactions. (3) As a result, the outermost layer of the binder is released into seawater.

[FIGURE 1 OMITTED]

Antifouling binder compositions

Polishing antifouling coatings can be divided into two types depending on the release mechanism of the binder material from the coating: "soluble" and "self-polishing" matrix coatings. In soluble matrix coatings, the binder contains slightly soluble parts. When in contact with seawater, the soluble pigment will generally dissolve faster than the soluble parts of the binder system, thereby forming a leached layer from which the soluble binder parts are released. The pigments continuously dissolve faster than the binder material, so the leached layer grows in thickness over time; eventually this will cause ineffectiveness of the biocide. (2) When rosin is used as soluble binder constituent in a soluble matrix coating, resinates of [Ca.sup.2+] and [Mg.sup.2+] may precipitate in the leached layer. (3) These must be physically removed by motion of the water, and therefore soluble matrix coatings are leaching considerably less biocide, when the ship is not in motion. (2) In general, adequate biocide release rates are obtained by a suitable matrix dissolution or reaction. Therefore, antifouling can in many cases be promoted by pigments or binders that increase the polishing rate without being biologically active. Relatively thick coatings (100-300 [micro]m) are necessary to retain antifouling effect for longer periods. (2)

The so-called self-polishing coatings rely on a chemical reaction between seawater and the binder material (e.g., the previously widely used tributyltin (TBT) methacrylate methylmethacrylate copolymers). In TBT-based coatings, the mode of action relies on hydrolysis of the ester bond, linking the organotin to the acrylic backbone. This unleashes the biocide, but it also alters the physical properties of the acrylic backbone, adding hydrophilicity and brittleness to the polymer, and when a suitable amount of ester groups have been hydrolyzed, seawater erodes the outer layer of the coating and a fresh layer is exposed. In this manner, hydrolysis of the ester bond controls both the release of biocide and the polishing of the coating. Seawater-soluble antifouling pigments are also added to these coatings to increase the antifouling effect, and the surface area of the binder. The steady-state thickness of the leached layer should be constant in most chemically reacting antifouling coatings. (2)

Seawater-soluble pigments

Polishing of the binder is caused by chemical or physical interaction between binder and seawater, and the surface area of the binder is therefore one factor affecting the rate of polishing. (4), (5) The leached layer effectively increases the surface area of the binder-water interface. As the specific surface area of the leached layer is determined by the soluble pigment that initially resided in the now empty pores, several pigment properties influence the polishing rate. Shape, particle size distribution (PSD), and rate of dissolution are pigment parameters affecting the rate of polishing, and pigment volume concentration (PVC) is a pigment-related coating parameter that can also be used to modify polishing rate. (6)

The influence of pigments on polishing has been addressed by means of mathematical models and experiments. (2), (4-7) For self-polishing TBT-based coatings, no polishing occurs (within 70 days at 30 knots) if the coatings do not contain water-soluble particles such as [Cu.sub.2]O and/or [ZnO.sup.4], and polishing rate is increased with decreasing particle sizes, and increasing PVC values. (6) In Kiil et al., (5) a mathematical model is used to screen for potential substitutes for cuprous oxide from a leaching and polishing point of view (i.e., not antifouling effect). According to the model, the dimensionless seawater solubility of the pigment, [alpha] ([alpha] = M * [C.sub.s]/[rho]) should lie between [10.sup.-6] and [10.sup.-8] in order to achieve polishing rates comparable to that obtained using cuprous oxide. As shown by Kiil et al., (5) the dimensionless solubility of most solids is much higher than [10.sup.-6], and of the compounds screened in the paper, primarily very toxic heavy metal salts have suitable solubility values.

For soluble matrix antifouling coatings, polishing rate is increasing with decreasing amount of insoluble binder; decreasing particle size, and increasing PVC of seawater-soluble pigments. (2) Mechanical stability of the coating is improved by adding insoluble pigments and binder constituents to the coating. (2)

Strategy of investigation

The aim of this work has been to substitute commonly applied seawater-soluble antifouling coating pigments for starch and starch degrading enzymes (glucoamylase). The starch is therefore added as an enzyme mediated water-soluble pigment. Successful implementation will provide an antifouling coating that can facilitate the release of an active antifouling biocide without the use of large quantities of heavy metal salts or oxides. The enzymes are only present in the coating in limited quantities, and only to facilitate degradation of the starch pigment. It is believed to have no antifouling active properties nor is it expected to reach the biofouling animals in quantities relevant for antifouling purposes.

In this work, different starches are evaluated for applicability in antifouling coatings. The best starch type is identified among 14 different starches obtained from rice, corn, and tapioca. Formulated coatings containing the novel pigment (spray dried starch with glucoamylase at the surface) are tested on a rotary set-up to monitor rate of polishing and leaching. It should be noted that neither starch nor glucoamylase should provide biofouling protection to the coating. These ingredients are only intended to provide polishing properties for the coating, and can therefore be used to partly or fully substitute cuprous oxide. Ideally, the starch and enzyme ingredient would be supplemented with an organic biocide to provide a metal-free antifouling coating.

Experimental

Starch

Starches were obtained from the companies Remy, Cargill, and Roquette. Table 1 shows the types of starches applied in the experiments discussed in this paper. Also provided in the table are the supplier information on average particle sizes and gelatinization temperatures.
Table 1: Source, name, and suppliers of the starches used

Name  Commercial   Source   Supplier  Gelatinization     Average
      name                             ([degrees]C)     equivalent
                                                      spherical volume
                                                         diameter
                                                        ([mu]m)

R1    Remy FG      Rice     Remy          65-73            2-8

R2    Remy B7      Rice     Remy          72               5

R3    Remygel      Rice     Remy          57               5
      663

R4    Remy DR      Rice     Remy          77               5

R5    Remyline AX  Waxy     Remy          65-73            5
      DR           rice

C1    C*gel        Corn     Cargill       62-71            15
      03401

C2    Clearam MH   Corn     Roquette      62-71            15
      0500

C3    Clearam MH   Corn     Roquette      62-71            15
      1015

C4    Clearam CI   Waxy     Roquette      62-71            15
      3000         corn

C5    Clearam CI   Waxy     Roquette      62-71            15
      1000         corn

C6    Clearam CH   Waxy     Roquette      62-71            15
      1505         corn

C7    HI-CAT       Waxy     Roquette      62-71            15
      21370        corn

T1    Clearam TJ   Tapioca  Roquette      59-70            20
      2015

Note: Whereas normal starch contains about 3/4 of the highly branched
amylopectin, waxy starch is 100% amylopectin. Gelatinization
temperatures refer to the temperature at which the starches form a gel


Particle size distribution

The PSDs of commercially obtained starches were measured using a Malvern Mastersizer 2000 from Malvern Instruments, and a Hydro 2000G sample disperser. The measurements were done on slurries of starch in ethanol to avoid dissolution of water-soluble components.

To establish the PSD of the most compatible starch type (i.e., C1) after it had undergone coating production, PSD of a full coating system was obtained. These measurements were done in xylene, and a few drops of the liquid paint were dispersed in commercial grade xylene, and PSD was measured directly on the slurry.

Critical pigment volume concentration

The critical pigment volume concentration (CPVC) was estimated by measuring the oil absorption (OA) according to the DS/EN ISO 787-5:1995 method. Equation (1) describes the relations between oil absorption and CPVC.

CPVC = [1/[[[OA.[rho](pigment)]/[100. [rho] (oil)]] + 1]] (1)

The oil absorption was established using commercial grade linseed oil. Approximately 1 g of powder was used, and linseed oil was mixed until a paste was achieved.

Water-soluble content of starches

The water-soluble content of starch-based ingredients was determined gravimetrically. Starch was mixed in deionized water and stirred effectively. The slurry was then centrifuged at 15,000 rpm for 10 min, and the dry matter content of the supernatant was determined gravimetrically.

Coating formulation

Previous studies have shown that starch compromises coating integrity, (8) and agents, such as fibers and insoluble pigments, must be included in the coating to provide sufficient mechanical strength over time. Fibers are known to reinforce the mechanical properties of coatings. (1) From the above described experiments, the starch-type C1 was found to be the most compatible antifouling coating ingredient (see sections "Results" and "Discussion"), and therefore only this starch type is considered.

The glucoamylase described by Dunn-Coleman et al. (9) was used in this experiment. Starch and glucoamylase were spray dried from water-based slurry. Seventy-five grams of starch was added to 500 mg of an enzyme solution of 0.6 glucoaymlase units/g slurry (1 glucoamylase unit corresponds to the amount of enzyme needed to produce 1 g of glucose in 1 h). The slurry was then spray dried. Air inlet temperature was 135[degrees]C, and powder outlet temperature was 80[degrees]C. At the spray nozzle, water cooling with 0[degrees]C water was used. The apparatus used was a Mini Spray Dryer B-191. In the slurry phase and during spray drying, glucoamylase binds to the surface of the starch granules at its starch binding domain (hydrogen bonding and van der Waals' forces). (10)

The binder constituents used to formulate the experimental coatings were chosen among the most common binder ingredients in Hempel's assortment of antifouling coatings for yachts. This was done to ensure that the experimental coatings designed did not deviate from well-known thoroughly investigated antifouling coatings in both the continuous and discontinuous phase, and thereby allow for mechanisms other than polishing and leaching giving rise to loss of coating material.

The binder system contained zinc resinate [produced from rosin (CAS no: 65997-06-0) and commercial grade zinc oxide], poly(vinyl methylether) (Lutonal M40, 45% from BASF AG), and acrylate [methyl methacrylate/n-butyl methacrylate/methacrylic acid terpolymer (molar ratio approximately 100:100:1) purchased as Degalan LP 64/12, from Rohm GMBH].

Zinc resinate was produced by adding a threefold molar excess of zinc oxide (technical grade) to highly hydrogenated rosin (CAS no: 65 997-06-0) dissolved in xylene (50 wt% rosin). After dissolving for 1 h, the slurry was left for 2 days, until infrared spectroscopy revealed a yield close to 100% yield. Zinc resinate was separated from the excess of zinc oxide by centrifuging for 2 h at 3000 rpm.

Initially, the intention was to mimic the binder composition of commercial antifoulings for yachts. However, screening tests (not published) showed that the high amount of starch was not compatible with a high content of hydrophilic poly(vinyl methyl ether), and a suitable binder composition was identified to contain roughly 60-70 vol% zinc resinate ([rho] = 1107 kg/[m.sup.3]), 15-25 vol% acrylate ([rho] = 1100 kg/[m.sup.3]), and 10-20 vol% polyvinyl methyl ether) ([rho] = 1050 kg/[m.sup.3]), and four physically dissolving binder systems were formulated within these ranges. The compositions of these are provided in Table 2. For each binder composition, one experimental coating containing starch and glucoamylase and two references were made. One reference contained starch with no enzyme, the other contained the common antifouling coating pigments zinc oxide and cuprous oxide in a ratio of 1:3. In addition to the experimental pigmentation, all the coatings contained 5 vol% fibers (Rock fibre MS603 from Brenntag Nordic), 5 vol% Iron(III)oxide (color pigment, Micronox H from Promindsa), and water scavenger and wetting agents in low amounts.
Table 2: The composition of experimental coatings

Binder    Zinc    Acrylate  Poly(vinyl methyl  Iron   Fibers
system  resinate                  ether)       oxide

A          39        15              6           5      5

B          39        12              9           5      5

C          36        15              9           5      5

D          36        12             12           5      5

Binder    Coating    Starch  Starch  [Cu.sub.2]O  ZnO
system  terminology           and
                             enzyme

A           AS         30
            ASG                30
            AR                           22.5       7.5

B           BS         30
            BSG                30
            BR                           22.5       7.5

C           CS         30
            CSG                30
            CR                           22.5       7.5

D           DS         30
            DSG                30
            DR                           22.5       7.5

Note: The amounts are given as volume percentages of dry coating.
A, B, C, and D denote the different binder compositions, S refers to a
starch reference, SG to the starch- and glucoamylase-containing
experimental coating, and R to the cuprous oxide-based reference


Water immersion test

Model coatings, composed of 39 vol% zinc resinate, 26 vol% acrylate, and 35 vol% starch, were applied on 5 cm x 10 cm polycarbonate panels with a wet film thickness of 300 [micro]m using a Dr Blade applicator. After drying for 1 week at ambient temperatures, the panels were immersed in tap water adjusted to 45[degrees]C. The panels were monitored gravimetrically during 5 weeks immersion. Before weighing, excess water was gently wiped off the panels. A commercially available coating (Mille Xtra from Hempel A/S) was included in the test series as a reference.

Polishing and leaching

Polishing and leaching characteristics were measured using a rotary set-up similar to the one described by Kiil et al., (4) except the temperature was kept around 35[degrees]C. The rotor was operated at 20 knots during the experiment. The pH was adjusted frequently to 8.2 using 1 M sodium hydroxide or 1 M hydrochloric acid.

Samples were prepared using overhead transparencies (3M PP2410) that had been primed using two-component (a polyamide adduct curing epoxy) Hempadur 4518 from Hempel A/S to improve adherence to the smooth transparency film. Coating samples were applied adjacent to each other using a Dr Blade applicator with a gap of 250 [micro]m. After curing, the coated transparency was cut in strips of 2 cm, resulting in samples of 1.5 -2 [cm.sup.2]. The strips were mounted on the rotor, and with frequencies of 14 to 30 days, samples were removed from the rotor, dried for 3 days at ambient conditions, and in order to distinguish the leached layer from the remaining coating (both red due to the insoluble iron oxide pigment), a thick blue line was drawn with a marker. The marker works as a leached layer indicator because the capillary pressure ensures penetration as far down as there are empty pores. The samples were cut in half and cast in paraffin, and the internal front of the sample was planed off before total film thickness and LLT was established using visible microscopy (coating cross-section inspection).

Blister box

The coatings containing starch and glucoamylase (i.e., the coatings ASG, BSG, CSG, and DSG) were tested for their tendency to blister according to ASTM D 4585. The coated surfaces of the panels were exposed to 40[degrees]C saturated water vapor, at an angle of 60[degrees] to the horizontal, and the back of the panels exposed to room temperature. The coatings were inspected every 2 weeks, and potential blistering was evaluated according to ASTM D714.

Results

Particle size distribution

The PSD of the starch types considered for use can be seen in Fig. 2. It is seen that the grain size of a starch depends on its source. Rice starches differ considerably from corn and tapioca starches that resemble each other very much. Considering the mean particle sizes (about 5 [micro]m) provided by the suppliers (see Table 1), it can be concluded that the rice starches shown in Fig. 2 have agglomerated. The applicability of rice starch as an antifouling coaling ingredient is therefore dependent on whether the agglomerates can be broken down and stabilized during production of the coating (not investigated in this work). Corn and tapioca starches are primarily distributed in the region between 10 and 20 [micro]m, and antifouling coatings of dry film thicknesses (DFTs) between 200 and 300 [micro]m can therefore contain these starches and still be considered homogeneous. The PSD of starch, and starch and glucoamylase when in a formulated coating, is shown in Fig. 3. In the figure, iron(III)oxide pigments are seen in the region around 1 [micro]m, (11) and the starch peaks are unaltered compared to Fig. 2, which shows the powder raw material as received from the supplier. The measurements were done several weeks after paint preparation, and it is therefore evident that corn starches, when in a coating, do not agglomerate, and stable liquid paints can be produced even with a considerable amount of starch in them.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Critical pigment volume concentration

The CPVC calculated from the oil absorptions measured is shown in Fig. 4. It is seen that all the rice starches, and the corn starch, C7, have critical PVC values just below 50 vol%. The CPVC values for the remaining starch types all exceed 55 vol%. However, the values are adequately high for all the starch types to be formulated into the coatings described in Table 2.

[FIGURE 4 OMITTED]

Water-soluble content

The water-soluble fraction of the different starch types is shown in Fig. 5. It is an important requirement for an antifouling coating ingredient that it contains little or no water-soluble material. High quantities of very seawater-soluble material in the film will draw water into the coating and cause blistering. It is seen that corn starches generally contain significantly lower amounts of water-soluble material than do rice starches. Especially the starch types C1 and T1 have low amounts of water-soluble contaminants; 0.12 [+ or -] 0.01 wt% and 0.13 [+ or -] 0.04 wt%, respectively.

[FIGURE 5 OMITTED]

Water immersion

The effect of water immersion on model paints is shown in Fig. 6. It is evident that the corn starch C1 has the lowest weight gain in water; in fact, the weight gain is close to that of the pure binder system. However, it is also seen that the coating still gains significantly more weight than the commercial reference does. This is due to the starch, which has a hydrophilic surface. The water uptake of antifouling coatings needs to be limited, otherwise swelling will compromise the mechanical integrity of the coating, and ultimately blisters can occur.

[FIGURE 6 OMITTED]

Due to its low amount of water-soluble contaminants, and the lowest water uptake of all the starch-types tested, the corn starch C1 ([C.sup.*]gel 03401 from Cargill) was selected as the most suitable starch type. All the subsequent results were obtained using this starch type.

Blister box

During 4 months exposure in the continuous blister box, none of the coatings tested developed blisters. This indicates sufficient mechanical stability to survive even prolonged exposure to seawater for all the coatings.

Polishing and leaching

The coatings based on the A-type binder did not polish to a measurable extent during the experiment. In case of the B-type binder system, starch and glucoamylase induce polishing, which is shown in Fig, 7. However, the dry film thickness increases initially due to swelling. Considering only the decreasing part of the figure (BSG), a polishing rate of 10.1 [+ or -] 0.3 [micro]m/month is derived. In the case of the C-type coatings, no polishing was detected during the experiment. All the D-type coatings swell considerably, which is seen in Fig. 8. However, after a delay, due to swelling, the coating DSG polishes with a rate of 7.8 [+ or -] 0.3 [micro]m/month.

Leached layers developed and reached stable values for all the coatings. Generally, the leached layers became relatively thick over time, ranging from 50 to 100 [micro]m after 100 days on the rotary set-up. However, other factors than leaching of water-soluble material may have affected the measurements of LLT. In fact, for the starch-containing coatings (not containing water-soluble material), false leached layer read-outs were seen in several cases. The measurements were very inhomogeneous, and therefore the data points have been omitted in Figs. 7 and 8. Also, for the starch-and glucoamylase-containing coatings, LLTs were very inhomogeneous. The LLT was identified using a marker, and the penetration of the marker, due to pores created by swelling and drying of the film, may have been the primary cause for the measured inhomogeneities. The LLTs measured are more than the desirable 5-20 [micro]m (for TBT-based coatings, see Kill et al. (4)) but may not be problematic.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Discussion

The weight gain reported for the model coatings in the water immersion test can only be ascribed to the different starch-types enclosed in the binder because only the type of starch varies. Figure 9 shows a correlation of water absorbed by model coatings against the oil absorption of the starches in the coatings. It is evident that two regimes exist. These two regimes are identified as covering starches containing either above or below 0.75 wt% water-soluble contaminants. There seem to be indications that when the content of water-soluble material is sufficiently high, this material is responsible for the vast majority of water absorbed by the coating. However, water uptake is not only decreased by reducing the amount of water-soluble content. Another factor is influencing the uptake of water, which may be the surface area of starch represented by the oil absorption in Fig. 9, but this is not a strong effect. It is therefore concluded that it is a requirement, but not sufficient, that starch used as antifouling coating ingredients must contain well below 0.75 wt% of water-soluble material. Notice also in Fig. 9 that data for rice starches appear to be more scattered than those for corn starches.

[FIGURE 9 OMITTED]

From the development of DFTs, it is evident that the coatings based on binder system A and C did not polish. This is due to the elevated content of acrylate that retards polishing. In the case where some acrylate was substituted for zinc resinate (coating BSG), smooth polishing was obtained. However, where some of the acrylate was substituted for poly(vinyl methyl ether) (coating DSG), polishing was delayed. This is due to significant swelling, which is seen when comparing the development in dry film thickness of coating CS (starch containing) with that of coating CSG (starch and glucoamylase containing). Poly(vinyl methyl ether) is more hydrophilic than the other binder constituents, therefore the coatings based on the C binder system take up more water, and swells more.

Polishing rates of 7-10 [micro]m/months were found, which is suitable for antifouling coatings for commercial ships. For yacht purposes, faster polishing is preferred. Faster polishing rates can be obtained by modifying the binder system, or the enzyme content of the starch pigment. If the enzyme activity is increased, the leached layer will develop faster, and polishing rates will be increased; conversely, lower enzyme activity will decrease polishing rates.

Figures 7 and 8 show that no polishing of cuprous oxide-based coatings occurred during the more than 20 weeks the polishing experiment lasted. This was probably due to the high amounts of retarders added to stabilize the coatings. However, it is expected that polishing of the copper-based coatings will occur eventually, when the leached layers have been allowed to develop sufficiently.

The leached layers were identified using a marker and factors other than pigment leaching may have caused the marker to penetrate the surface of the film. One may be the open pores created during swelling and subsequent drying of the coating. Another may be a result of the heterogeneous leached layers of the coatings containing starch and enzymes. This may be a result of endocorrosion by the enzymes. Digestion of starch granules by glucoamylase may either be via peeling off the outer surfaces layer by layer (exocorrosion), or by drilling tunnels into the starch granules (endocorrosion). (12) If endocorrosion occurs in the coatings, this will mean that the porous leached layer (identified by the marker) is not necessarily depicted of starch. Well defined, empty pores in the leached layer are characteristic for cuprous oxide based coatings. (4)

The fact that measurements were done on dry coatings, and that the samples were intact, shows that the coatings formulated may undergo wetting and drying without cracking.

Conclusions

In this paper a suitable starch for use as antifouling coating ingredient has been identified. It has a very low content of water-soluble material, and only little water is taken up by coatings containing the corn starch, [C.sup.*]gel 03401. The PSD of the starch type is also suitable for inclusion in an antifouling coating.

Soluble matrix coatings can be formulated so that they polish due to the enzyme-mediated release of the starch pigmentation in the form of dissolved glucose. In fact, binder compositions that did not polish based on a high content of cuprous oxide, polished when the cuprous oxide was substituted for starch and glucoamylase. Polishing rates in the region of 7-10 [micro]m/month were found, and whereas the polishing rates achieved here are suitable for commercial ships, they should be increased to meet yacht purposes.

Enzyme activity in the coating may be altered to change the polishing rate of the coating, and the coating composition should be optimized to minimize the adverse effects of the starch. Furthermore, the starch and enzyme pigmentation should be tested in other binder systems.

Acknowledgments This work was funded by the Danish Ministry of Science, it is part of the CHEC Research Center funded a.o. by the Technical University of Denmark, the Danish Technical Research Council, the European Union, the Nordic Energy Research, Dong Energy A/S, Vattenfall A.B., F L Smidth A/S, J.C. Hempel's Foundation, and Public Service Obligation funds from Energinet.dk and the Danish Energy Research program.

References

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S. M. Olsen, L. T. Pedersen

Hempel A/S, Lundtoftevej 150, DK-2800 Kgs. Lyngby, Denmark

S. M. Olsen, K. Dam-Johansen, S. Kiil (*)

Department of Chemical and Biochemical Engineering, Technical University of Denmark, Building 229, DK-2800

Kgs. Lyngby, Denmark

e-mail: sk@kt.dtu.dk

J. B. Kristensen

Genencor, Danisco A/S, Edwin Rahrs Vej 38, DK-8220

Brabrand, Denmark

J. Coat. Technol. Res., 7 (3) 355-363, 2010

DOI 10.1007/s11998-009-9191-3
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Author:Olsen, S.M.; Pedersen, L.T.; Dam-Johansen, K.; Kristensen, J.B.; Kiil, S.
Publication:JCT Research
Date:May 1, 2010
Words:5248
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