Miljøprojekt, 910 – Substitution af biocider i bundmaling til skibe med enzymer

Substitution of Biocides with Enzymes in Antifouling Agents for Ships

Summary and conclusions

Biofouling

All surfaces in nature will be fouled after shorter or longer periods by many different organisms (“biofouling”). In the sea fouling is especially aggressive, and therefore fouling of ship hulls is a very big problem, which is very costly for the ship-owners to solve or limit. In the EU the costs of antifouling are calculated to be one billion Euro per year, and globally the costs are estimated to Euro 6.6 billion per year. If the fight against fouling of a ship hull is not successful, it will result in reduced speed of the ship, loss of manoeuvrability and increased consumption of fossil fuels and, thus, increase of the greenhouse effect. Besides, fouling will also have a tendency to increase corrosion of the ship hulls. As many yacht owners know, if nothing is done, soon small barnacles and algae will cover the ship hull. The character of the fouling and the speed of its progress depend on where in the world the ship is sailing – but no matter where, even in our cold regions a yacht will be fouled within few months.

The use of TBT was banned for yachts (<25 m) in Denmark in 1991. In the International Marine Organisation (IMO) it was agreed that TBT is harmful to the environment, and this compound should be removed from all ships. The IMO has therefore recommended a programme for removal of TBT on ships: Since 2003 TBT must not be applied on ships, and from 2008 it must be removed from all ships or sealed. The story about TBT has resulted in an increasing awareness about biocides in general and has resulted in a series of national and regional restrictions mostly starting on the yacht market. It must be noted, though, that the IMO convention will not enter fully into force until a minimum of countries has ratified it. Anyhow, the EU has decided to implement the ban of TBT without waiting for the ratification of the convention. So, the 2003 ban is in force in the EU and for EU ships.

In Denmark, regulations have been imposed in 2000 on import, sale and use of antifouling coatings for yachts. This regulation banned the use of Irgarol and Diuron. The latest regulation in 2003 restricts the use of copper compounds by limiting the leaching per surface area.

Non-toxic antifouling

The industry has now realised that it must develop alternative antifouling technologies, as the existing toxic biocides all will be banned sooner or later. In the meantime the most frequently used biocide to replace TBT is expected to be copper compounds, but Denmark and Sweden have already introduced restrictions on the use of copper compounds on yachts. All over the World many trials are being performed to find alternatives. For the big ships two alternatives will be used most commonly in the coming period:

One method is coatings with low surface energy properties resulting in very smooth, non-sticky surfaces, the so-called “fouling release” surfaces. In these technologies silicone and teflon compounds are used. Most used today are the silicones. These coatings are used for ships sailing very fast (15-20 knots) and sailing constantly with only short breaks. The silicones are very expensive compared to other products and more difficult to apply and repair. Judging alone from the low surface energy values, teflon coatings should be smoother than silicone coatings. But it has been shown that a silicone surface is smooth due to the constant release of low molecular silicone compounds resulting in a “soapy” surface. The environmental impact of these compounds has not yet been investigated.
Most other alternatives within antifouling focus on finding “non toxic biocides”, i.e. organic compounds which are toxic for the target organisms on the surface of the ship hull, but are degradable when released into the seawater and, thus, do not accumulated in the environment like TBT and copper.

Enzymes

Enzymes are proteins and are necessary for all life. These molecules are soluble in water and are easily degraded in nature into carbon dioxide and water and are natural ingredients in the ecological cycle. Enzymes are organic catalysts, i.e. they can transfer one product into another without being consumed in the process. So, enzymes can be used to hydrolyse (break down) the molecules that marine organism use for their attachment to a surface. It is known from literature that these attachment compounds (glues) contain protein and polysaccharides. It was therefore obvious to investigate if proteolytic enzymes could be used in antifouling instead of the toxic compounds used today, like copper. The concept of using enzymes as antifouling compounds is based on a patented technology developed by BioLocus.

The compatibility of enzymes with paint binder systems

First, experiments were performed to investigate if the enzymes could be dispersed into paint binders normally used in the marine paint industry, and still retain their activity in the mixture. The enzyme Alcalase, a serine protease from Novozymes A/S, was chosen as model and was tested with seven different normally used binders. This investigation revealed that the Alcalase is still active when mixed with modified rosin, hydrogenated rosin and polyvinyl acetate emulsions. No activity was measured with polyvinyl chloride, acrylics and silicone binders.

Based on these results, two prototype paints were produced – a water based and an organic solvent based with different combinations of enzymes. These formulations were tested on rafts in a sailing season in two Danish yacht harbours, Jyllinge and Helsingør. It turned out that the fouling on the solvent-based paint formulation was significantly smaller than on the waterborne paint, and it was especially effective against barnacles.

Paint formulation with enzymes

Combinations of rosin, acryl and enzymes have been investigated in relation to viscosity, water uptake, application, paint film properties and leaching of enzyme from the paint.

In general the viscosity increases when adding the enzyme to the formulation. The water uptake can be controlled, but adding the enzyme does not change the self-stability of the final formulation.

Otherwise the paint properties have not been changed significantly compared to paint without enzyme.

Leaching of enzyme from the paint

Investigations of the leaching of the enzymes from the paint have been performed in laboratory experiment. These investigations resulted in the conclusion that the enzymes are still leaching from the paint after 31 days. Alcalase analysis suggests that activity can be found after three months, but the activity is reduced considerably and the amount of enzyme on the paint surface is probably dependent on the transport from the lower part of the paint.

Eco-toxicology effects

The three enzymes used in the raft field experiments, Alcalase, AMG and Pulpzyme, have been investigated for eco-toxicology effects. The investigations could be concluded as follows:

All three enzymes were 100 % biodegradable within 12 days in seawater. The criterium for biodegradability in seawater (60 % of ThOD) was achieved within three days.
Alcalase showed inhibition of the growth rate of Skeletonema costatum with an EC50 of 17 mg/liter and a NOEC of 5 mg/liter.
The mixture of the three enzymes showed inhibition of the growth rate of Skeletonema costatum with an EC50 of 45 mg/liter and a NOEC of 10 mg/liter.
Alcalase and the mixture of the three enzymes showed no acute toxic effects on Daphnia magna and Danio rerio in the tested concentrations 1-100 mg/liter.
The enzymes AMG and Pulpzyme showed no acute toxic effects on Skeletonema costatum, Daphnia magna and Danio rerio in the tested concentrations 1-100 mg/liter.

Based on the achieved results it can be concluded that the tested enzymes shall therefore not be classified as hazardous to the environment according to the Commission Directive 67/548/EEC concerning classification of dangerous chemicals.

Filed test on yachts and rafts

In the last year of the project period (2003) the Danish Sailing Association had selected 19 yachts in five different harbours in Denmark. The selection was based on criteria to secure a representative sample from "Danish waters" in terms of biofouling and salinity.

The test was performed with paint delivered by Jotun A/S. Two types were used, a self-polishing paint: AF 2022 and a hard paint: AF 2047. Both with organic solvents and the enzyme used were Alcalase.

The yacht owners were asked to participate in the trial applying the test paint on their yacht and to fill in a questionnaire form – one after applying the paint and one form at the end of the trial. There were two test paints - a self-polishing and a hard paint both containing the Alcalase. One quarter of the yachts in each harbour were painted with the hard paint and the rest with the self-polishing paint.

All yachts were initially painted with a 1 component Jotun primer before applying the test paint to secure no interference from the old paint already on the bottom. A copper control strip was also applied at the centre of the yacht with Jotun “Non-Stop”.

In the same harbours rafts containing the same paints as on the yachts were also immersed.

The participants were asked to fill in two questionnaire forms, one with application performance, and one at the end with antifouling results. 95 % of the participants filled in the two questionnaire forms.

Results from field test

Three inspections on location were performed during the test period – one in June (seven yachts) one in August/September (eight yachts) and one in October/November (all yachts).

Application:

Both test paints were according to the owners difficult to apply – the self-polishing being the most difficult.

Antifouling performances:

During the test period some of the owners had washed the bottom of the yacht due to fouling.

This is considered to be a drawback for the evaluation of the efficiency of the paint. Those yachts not being washed during the period were considerably more fouled than yachts being washed between inspections.

Five out of 19 test yachts left the trial during the period – one sold. The rest of the test yachts that had left the trial were yacht racers.

Conclusion

Based on the inspections performed and the reply from the owners the following can be concluded for the two test paints:

Fouling comes quickly with slime and algae. Owners observed more fouling in the waterline earlier than they normally do.
Sailing speed is reduced in comparison to normal, and sailing performance also reduced. Owners are in general not satisfied with the sailing performance.
Four owners decided to leave the trial due to fouling – all yacht racers.
The self-polishing paint is washed away too quickly, but in spite of that they have a better antifouling performance than the hard paint.
Barnacles are only observed in small amounts on the test paints except in one harbour (Horsens 50 –100% Barnacles).
Cleaning of the bottom for fouling was done for the major part of the participants. During the cleaning paint was washed away. This has probably reduced the amount of remaining paint to a low level and thereby reduced the fouling efficacy.
On the non-washed yachts (especially the yachts with self-polishing paint) the main part of the paint was washed away as a consequence of normal sailing. This is also the case for yachts applying two layers of the paint. The polishing performance is therefore not good enough.

It can therefore be concluded that the test paints in the tested formulations do not fulfil the requirements of the yacht owners to secure a reasonably fouling-free bottom and satisfactory sailing performance.

Perspective

It has not been possible within the time frame of the project period to develop a biocide-free alternative to the existing toxic commercial paint products.

The results from the present investigation indicate that it is possible to develop an enzyme-based antifouling product.

The quality of the paint formulation can and must be improved. The self-polishing paint was polished off too quickly, and the enzymes were washed out of the paint at a too high rate, which was also demonstrated in the laboratory experiments.

It is important, however, to develop methods to retain the enzymes in the paint for a longer period. This can be done by immobilisation of the enzymes to the binder molecules or by altering the properties of the enzymes in order to reduce their hydrophilic character.

It is important to conclude that the ecotoxicology studies showed 100 % biodegradability within 12 days and no acute toxic effects on algae, daphnia and fish.

The Danish Sailing Association regard the enzyme-based paint technology a promising biocide-free alternative. The results of the test show that the technology should be improved to increase the antifouling performance, especially towards slime, algae and bryozoans, and to increase the polishing performance of the paint.

Raft tests performed in parallel to the yacht test show that enzymes in other formulations/combinations than the present yacht test had a higher potential to reduce the biofouling.

The Danish Sailing Association therefore hopes that work to improve the technology continues to the benefit of the environment and the sailors.