Testing Materials

Coatings with low/non-toxic active ingredients

Field testing of some preliminary candidates in CRAB indicated some potential but more research is needed. There are some products on the market incorporating natural antifoulants but they are rarely used due to high prices. Furthermore, their efficacy is not clear. A selection of non-commercial candidates has been field tested in CRAB. The general outcome is that the performance of these candidates is not very good. It must be remarked that the tested systems are still under development.

Fouling-release coatings

This type of coating works on the principle that fouling does occur on the surface but due to low bioadhesion is easy to remove. Fouling-release or non-stick coatings have a low surface energy and most products on the market are silicone based. The coatings do not contain active ingredients.

The fouling release properties of silicones are mainly attributed to their “non-wettable� surface (water doesn’t form a surface film but rather falls away from the surface (like beads of water on a freshly waxed car). They work, not so much by stopping fouling in the first place- but by reducing adhesion strength so that the organisms are readily attached under flow- e.g. when a ship or boat starts to move in the water.

Left and middle: Fouling does occur on non-stick or fouling-release coatings but can easily be removed. Right: application of silicone fouling-release coatings is through dipping.

Field testing at several CRAB sites showed that most of the silicone products performed well throughout the 2-year test period: fouling did accumulate on the silicone treated netting, but at a slower rate than non-treated control netting. More importantly, the fouling was very easy to remove.

Comparison of netting coated in silicone paint and non-coated (control) netting immersed at Sagres for 22 months.

At present, silicone is a costly alternative to copper based treatment. It is hoped that the higher cost of silicone may be offset against an increased duration between applications or by increasing the life of a coated net. At current costs one application would have to last anywhere between 2 - 10 years, based on copper based antifouling being reapplied every 6 months. However at the moment this seems unlikely considering the cracking in the coating that occurs with loading. Additionally the weakening effect of solvent in the silicone is likely to decrease the nettings useful lifetime.

Spiky coatings

A spiky coating with protruding "needles" (0.2 - 2 mm) deters certain types of fouling. Although currently not in commercial use, a prototype net product is under test in Turkey (more information can be found at www.micanti.com). In CRAB, a broad range of fibre types and densities has been developed and evaluated.

Left: Nylon netting on a roll, ready to be used for net production. Middle: detail of a blue fibre coating on nylon netting. Right: fish net cage treated with blue fibre coating.

The outcome of the CRAB field testing was that the coatings were not sufficiently effective to take care of the broad fouling spectrum. However, the coatings do, to some extent, reduce specific types of fouling such as barnacles and tubeworms and are therefore potentially suitable for specific regions or applications.

More detailed descriptions are available in the CRAB Project Best Practice Guidelines.

Descriptions of other strategies still undergoing development can be found through the Strategy page of the eLearning section on this website.