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Foul-release coatings are considered environmentally friendly. These coatings have two basic
mechanisms to protect against fouling; the hydrolysis of polymers which (a) removes fouling
with the eroded coating layers and (b) minimizes the initial attachment and strength of the
attachment through the properties of the coating surface, preventing or allowing only weak
adhesion of the organisms to the surface (Callow 1993, Wells and Sytsma 2009, Nair 1999). In
order to adhere to surfaces, fouling organisms synthesize and secrete proteinaceous adhesives
that secure them in place. Materials with low surface free energy offer low adhesion strength,
resulting in poor attachment (Nair 1999).
Foul-release coatings differ in chemical formulations and the type and amount of free oils and
other additives. Many foul-release coating systems require a duplex system, which is multiple
layers with different properties applied to the substrate in order to improve adhesion and
corrosion protection. Awater-resistant anticorrosive layer (epoxy) protects the base substrate and
augments the adhesion of the topcoat. Various tie coats bond the tough bottom layer to the
hydrophobic topcoat. Catalysts, solvents and curing times improve adhesion (Wells and Sytsma
2009). Most of the topcoats have properties like low surface energy, non-polarity and elasticity,
giving the coating a very smooth, slippery, low friction surface. A low surface energy coating
has the lowest incidence of fouling. The best low energy surfaces were dominated by closely
packed methyl groups and characterized by a surface tension of 22 mN m
-1
(22 dyne cm
-1
)
(Callow 1993).
Efficacy varies by target species. Hydrophilic surfaces are more effective against proteins and
cell adhesion, while hydrophobic surfaces are more effective against macrofouling (Wells and
Sytsma 2009). Barnacles and bryozoans tend to foul heavily on low energy surfaces (Callow
1993)
5.2.1.1.
Silicone
Silicone elastomer foul-release coatings provide a durable, ultra smooth, slippery, hydrophobic
surface. The rubbery nature of silicon causes a weak bond that fractures to help dislodge or
remove organisms under flow conditions (Wells and Sytsma 2009). Some silicone coatings
contain petroleum oils, which leach out of the coating over time tomaintain a slick surface, and
enhance the low surface energy characteristics of these coatings. The amount of oils used and
thickness of the coating film directly affect the effective service life of the coating (Mussalli
1989). The application thickness of silicone coatings is typically 150 µm. This thicker coating
helps control the coating modulus because less energy is required to fracture the bond between
the foulant and the coating (Chambers et al. 2006).
Silicone based foul-release coatings appear to function better than fluoropolymers (described in
Section 5.2.1.2.), even though fluoropolymers have more mechanical strength. Silicone foul-
release coatings are very promising in static and dynamic conditions, but they are soft and not
very abrasion or gouge resistant. They have high erosion resistance compared to epoxy coatings
for sediment and silt ladenwaters, but they don’t work as well in areas exposed to heavy debris
impacts (Skaja 2012).When tested in seawater, these coating performed satisfactorily for two or
more years (Baier andMeyer 1992). Velocity has the effect of shearing off the foulant organism
from the coating (Mussalli 1989).
