Silicone PDMS

This type of coating gives surfaces that often exhibit a "low surface energy". This means that when water is applied they form beads/droplets and easily run from the surface. This can be translated into a surface that is easily cleaned. All existing fouling-release coatings are based on silicone polymers (PDMS). Application is critical (and difficult). Mechanically not very strong (rubber like). All commercial systems (shipping) are silicone based.

Methodology

The application of these coatings on flat surfaces such as boat hulls and shellfish trays requires a 3-component system to achieve good adhesion to the substrate. This means that a primer, tiecoat and topcoat are needed. However, for nylon netting, the primer and tiecoat are not required.

Key Factors Positive

- Fouling does accumulate on the silicon treated netting but for some products at a slower rate than non-treated control netting.
- Fouling does occur on these non-stick or fouling-release coatings but can be easily removed. Some products have also an inherent antifouling performance, i.e. lower fouling build-up than on reference surfaces. The best systems are "self cleaning" when fouling load is heavy.
- Cleaning of silicone coated trays can take up to five times less than cleaning of standard trays.
- Since silicone is non-toxic, registration within the BPD is not required.
- Washings from silicone coated nets should not require processing before discharge.

Key Factors Negative

- Mechanical strength tests of the netting revealed that some ingredients in the silicone paint can actually weaken the netting.
- The stiffness or lack of flexibility of the netting may increase labour costs in the handling of coated nets during changes or cleaning.
- Sensitive to moisture (careful cleaning and drying is essential prior to dipping).

Cost Benefit

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.

Conclusions/ Discussion

Until the use of copper based paints is phased out (either at a European or a global scale), and there are improved economies of scale in production and supply of alternative coatings, it is unlikely that silicone coatings will be adopted because of their high cost and application issues.