Water Journal : Water Journal March 2011
sewer processes refereed paper 74 MARCH 2011 water SULFURIC ACID PERMEATION IN EPOXY MORTAR COATINGS A comparison of various commercial epoxy mortar coatings challenges conventional wisdom. Abstract The performance of various commercially available epoxy mortar coatings was compared by measuring their acid diffusivity. The results showed that acid diffusivities increased with coating thickness, due to reduced polymer alignment and tortuosity in both polymer and filler, which challenges the notion that greater coating thicknesses provide greater protection or environmental barrier. This has significant implications to the current selection of coatings and sewer protection. Introduction For the last 30--40 years water utility industries have trialled various methods of rehabilitating and protecting sewer pipes from sulfuric acid corrosion at the crown and beach levels (Nixon 1997, Sand & Bock 1991). Coating the surface with epoxy mortars as a first line of defence has been used widely within the industry. Successful and durable rehabilitation of sewers, however, has proven extremely difficult. There are significant technical gaps in predicting their performance and durability in sewer environments, a specific one being the relationship between epoxy formulations and coating performance. The lack of national coating specifications or standards for selecting coatings for sewer application in Australia is associated with the fact that much of this knowledge is proprietary and differs between manufacturers. Greater fundamental understanding of the effects of coating formulation on their performance is essential in developing rigorous sewer protection strategies. Epoxy does not refer to a specific material, but to a broad family of co- polymers of an epoxide resin and hardener with differing physical and chemical properties. Important features that impact on its durability include curing agents (for example, amine, amide, cyclo- aliphatic), solvents and fillers. It is known that epoxies generally have excellent mechanical properties and high chemical resistance (Liu et al. 2004). However, fully cured epoxy resin formulations are brittle and have high water uptake (Liu et al. 2003). Remarkable reduction in the permeabilities of polymeric coatings to gas and liquid eluents, in addition to improved physical properties, including strength and abrasion resistance, has been associated with the addition of inorganic fillers (Alexandre et al. 2009, Herrera-Alonso et al. 2009, Lu et al. 2005, 2007, Ray et al. 2003 and Sun et al. 2008). Much of this work has focused on diffusion of gases and moisture. However, the specific nature and properties of the epoxy and fillers that would retard acid diffusion, specifically those found in the sewer, is currently not well understood. The Study In this study, the effects of coating thickness and filler properties on the sulfuric acid permeation were investigated. Although other organisms and bio-acids are present in the sewer, it is recognised that much of the sewer corrosion, specifically for concrete, arises because of the presence of the Thiobacillus strain and its biogenic acid (Aviam et al. 2004, Yamanaka et al. 2002). The relative performance of coatings as an environmental barrier was measured from their sulfuric acid diffusion coefficients. The diffusion of acid into cured epoxy resin is generally considered to obey Fick's law (Crank 1956). The solution for Fick's law for short times is reduced to the following equation for the initial stage of diffusion (Liu et al. 2008): M Valix, H Bustamante This paper is based on the presentation at the IWA Conference, Sewer Processes Network 6, November 2010. Figure 1: SEM photograph of coatings A, B, C and D.
Water Journal April 2011