Environmental Engineering Reference
In-Depth Information
Foul-release coatings are environmentally friendly and operate by two basic
mechanisms: the hydrolysis of the polymer surface that 1) removes fouling with
the eroded coating layer, and 2) minimises the initial attachment and the strength
of attachment because of the properties of the coating surface. Coatings based on
these foul-release mechanisms are effective and reduce the initial settlement and the
strength of attachment. They are mechanically weak and are subject to failure because
of detachment and abrasion. Heavy metal-based coatings are effective, and work
by releasing biocides such as copper into the surrounding water, which may impact
on the other lora and fauna. Coatings such as coal tar, epoxy resins or other anti-
corrosion, anti-abrasion agents are not cost effective to use against mussels. Non-toxic
coatings that rely on low-surface tension to create smooth/slippery surfaces, use of
non-metal fouling repellants in traditional coatings, non-toxic foul-release coatings
(ablative hydrophilic polymer ilms and low free surface energy ilms), and thermal
spray coatings (in which slow dissolution of metal ions repels fouling organisms) are
some current research areas. Extracts of
Pseudomonas
sp., incorporated into paints
show good antifouling activity against bacteria, the
B. amphitrite
barnacle cyprid,
and
Ulva lactuca
algal zoospores. A combination of neem oil/linseed oil treated Nylon
ishing nets showed reduced macrofouling when compared to untreated nets after
20 weeks. Foul release agents such as PDMS, chitosan and polyvinyl pyrrolidone are
incorporated into this concoction to give an antifouling mixture [8, 29−31].
4.7 Bioadhesive and Bioinert Surfaces
In order to surface engineer a substrate so that it turns into a low or non-sticking/
non-adhesive one, the mechanism of bioadhesion needs to be understood. Strong
hydrophilic, low energy surfaces appear to be promising for industrial and marine
applications. Such an approach is also ecologically friendly. The adhesion bonding
involves chemical (such as acid/base Lewis) and electrostatic interactions, Lifshitz − van
der Waals forces, mechanical interlocking, diffusion and so on [32]. A thermodynamic
approach considers the adhesion of bacteria to a surface as an equilibrium process
and when the substrate is more hydrophobic the system is far away from equilibrium.
Pedri [33] considered only dispersion forces to calculate free energy but a model
which is based on the concept of 'theta surface' is later introduced which deines a
critical surface tension range of 20−30 mN/m. Materials that are designed for strong
bioadhesion should not be in this range and those that require easy foul release should
be in this range ('theta surface'). Macrofooulers including barnacles, mussels algae
and so on, make use of the concept of bioadhesion to bond to variety of wet surfaces
in saline and turbulent conditions.
The well-known 'Baier curve' relates surface energy (water contact angle) or surface
tension to the relative amount of bioadhesion. It indicates that the bioadhesion is
