Coatings World - April 2013

Figure 1. Homogeneous coating surface without defects ( 3 months exposure). Figure 2. Algal colonisation on the coating (also at 3 months exposure).

Figure 3. Fibre ‘pop out’ through the coating.

are subject to harsh environments, dynamic loads, continuous expansion and contraction by heat, rain/seawater splash, impacts from debris, erosion, micro-organisms etc. In this condition, most coatings deteriorate in a short period of time in the form of cracking, blistering, disbanding or chalking. The application of the TiO2 based coating tested in this research was not designed to defend from microbial growth from ‘within’ the concrete, effectively a living substratum, observed in figure 7. The occurrence of a bacterial biofilm formation under the coating has significantly effected the performance of the coating. A study into the addition of TiO2 powder with an average size 21 nm (30% rutile and 70% anatase) into a bacterial colony, showed that 60–120 min were sufficient to destroy all the bacteria ( 7). Other workers also confirm that using lower dimension TiO2 particles leads to a faster bacterial destruction ( 8). These new observations of bacterial growth seen in figure 6 are detrimental to the long term durability of the coating and requires further investigation. This newly observed degradation mechanism, see figure 8, reported here, of a coating has implications for not only the construction sector.

Conclusions

Based on the analysis conducted, the following conclusions may be drawn that macro and micro synthetic fibres at the surface of concrete inhibit a strong and durable bond between the coating and the substratum, accelerating cracking and the eventual breakdown of the coating. Algal filamentous growth including diatoms attached to the surface of the coatings, applies further pressure on the integrity of the coating. Bacterial filamentous growth from within the matrix of the concrete, grows at the coating/concrete interface. This growth disrupts the bond between coating and substratum, leading to the de-lamination of the coating. Based on the results presented, further research is recommended to consider factors such as microbial growth under a coating, application methods and variation, coating composition, and long term durability. Furthermore, research in this field, needs to be developed to determine if any coatings have the potential to be effective in the long term strategy against marine biofouling. CW

Peter Hughes is a final year PhD student at the University of Central Lancashire, UK, investigating marine biofouling and its implications for the durability of marine concrete.

Acknowledgement

The author thanks his supervisors for their guidance. D. Fairhurst, Professor I. Sherrington, Dr. N. Renevier, Professor L.H.G. Morton, Professor P. C. Robery and Dr. L. Cunningham.

Further discussions are invited at: PHughes1@uclan.ac.uk

References

1. Fujishima, A., Rao, T., Tryk, D. Titanium dioxide photocatalysis. Journal of Photochemistry and Photobiology. 1, 2000, 1-21.

2. Peller, JR, Whitman, RL, Griffith, S, Harris, P, Peller, C, Scalziatti, J. TiO2 as a photocatalyst for control of the aquatic invasive alga, Cladophora, under natural and artificial light. Photoch. Photobio. A. 186, 2007, 212-217.