Coatings World - April 2013

undercoat was applied after a drying time of 24h. Then, three separate topcoats were applied, each 10µm and a further 24h drying time, thus, bringing the overall thickness of the photocatalytic coating to 50µm. Although the coating was composed of a number of different layers, the comparatively short time between applications ensured that the finished complete layer did not show any distinct separate layers, but can be treated for all intents and purposes in this research as a single layer, see figure 1.

Furthermore, the attachment of fila-
mentous algae to the surface of the coat-
ing, seen in figure 4, was also observed to
be detrimental to its long term durability.
Diatoms, illustrated in figure 5, form an-
other component of marine biofilms and
act a settlement mediator for larger foul-
ing. The diatom attachment to coatings
examined showed that this single algae
cell accelerated coating degradation.

Results And Discussion

The biological complexity of the phenomenon, part of a larger study ( 4), referred to as marine biofouling, is enormous. It has been shown here and in previous research ( 5) that it is an ecological community with entities originating from all that we call life. Also, each organism has its own solution for how to find and attach on a surface, evolved during millions of years. It is the author’s view, it is impossible to invent new antifouling coatings without restricting the problem, meaning that several antifouling strategies have to be part of a holistic approach, leading to a bigger solution.

There are however several obstacles to be cleared before titanium dioxide photocatalyst technology can be adopted in the control of marine biofouling. Not only the fact that the applicability of this technology is limited considerably because the catalyst works only where there is light, but the application of a coating to composite materials such as concrete has limitations, as shown in figure 2.

The performance of the coating was observed to be heavily dependent on the underlying composite material. First of all, due to differences in intrinsic properties the synthetic fibres, in abundance at the surface of the concrete samples, inhibited a satisfactory bond between coating and substrate. The thermal coefficients of the coating were different from that of the concrete and its constituents. Thermal expansion and movement, referred to as ‘fibre pop out’, of exposed fibres instigated a cracking of the coating, as seen in figure 3, resulting in not only reduced photocatalytic activity but also structure and strength destructions.