Water Journal : Water Journal March 2011
refereed paper sewer processes water MARCH 2011 77 the tortuosity provided by the filler. Lu and Mai (Lu et al. 2005) proposed the following simple tortuosity model: (3) Examination of our results, however, show the diffusion coefficients are lowest when the concentration of filler is low (~12%) and highest when the filler concentrations are high (>40%) (see Figure 6). This appears to contradict the tortuosity model proposed by Lu and Mai (Lu et al. 2005) This effect is most prominent as the coating thickness increases, which could suggest issues with filler dispersion. Intuitively increasing the concentration of the filler should provide a greater barrier to acid permeation. However, substantially increasing the concentration of the filler could induce the clustering of the filler and, in effect, increase the overall particle size and reduce tortuosity (see coatings A and D in Figure 2). The effect of the shape or the aspect ratio on the diffusion coefficient was not evident in this study; however, it is clear the reduced particle size of the filler resulted in lower acid permeability (see Figure 7). Conclusions The following conclusions were derived from this study: I. The tortuosity of composite epoxy coatings was imposed by polymer alignment and filler properties including filler size, dispersion and concentration. II. Thicker coatings did not necessarily provide greater protection. The alignment of the polymer chains, which lent itself to greater tortuosity, appeared to control the permeation through the coating. Greater chain alignment was characteristic of thinner coatings and this affected higher tortuosity and lower acid permeability. III. The filler concentration in effect provided greater tortuosity. However, there was a limit to the concentration that would promote an effective barrier. Increasing the concentration of the filler promoted greater clustering, which appears to have resulted in larger particle size and reduced tortuosity. I V. There was little evidence of the effect of the shape or aspect ratio of the filler. However, reducing the particle size of the filler was found to significantly reduce the permeability of sulfuric acid. Acknowledgements The authors would like to acknowledge H Mineyama, WH Cheung and J Shi from the School of Chemical and Biomolecular Engineering, University of Sydney, who performed the experiments and analytical testing of the samples for this study (email: email@example.com; firstname.lastname@example.org, email@example.com). The Authors Dr Marjorie Valix is Associate Professor at the School of Chemical and Biomolecular Engineering, University of Sydney (email: firstname.lastname@example.org). Dr Heri Bustamante is Project Manager at Science & Technology (Sydney Water), PO Box 399, Parramatta, NSW 2124 (email: Heri. 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Water Journal April 2011