A Critical Review of the Time-Dependent Performance of Polymeric Pipeline Coatings: Focus on Hydration of Epoxy-Based Coatings

Abstract

The barrier performance of organic coatings is a direct function of mass transport and long-term stability of the polymeric structure. A predictive assessment of the protective coating cannot be conducted a priori of degradation effects on transport. Epoxy-based powder coatings are an attractive class of coatings for pipelines and other structures because application processing times are low and residual stresses between polymer layers are reduced. However, water ingress into the polymeric network of these coatings is of particular interest due to associated competitive sorption and plasticization effects. This review examines common analytical techniques for identifying parameters involved in transport in wet environments and underscores the gaps in the literature for the evaluation of the long-term performance of such coating systems. Studies have shown that the extent of polymer hydration has a major impact on gas and ion permeability/selectivity. Thus, transport analyses based only on micropore filling (i.e., adsorption) by water molecules are inadequate. Combinatorial entropy of the glassy epoxy and water vapor mixture not only affects the mechanism of membrane plasticization, but also changes the sorption kinetics of gas permeation and causes a partial gas immobility in the system. However, diffusivity, defined as the product of a kinetic mobility parameter and a concentration-dependent thermodynamic parameter, can eventually become favorable for gas transport at elevated temperatures, meaning that increasing gas pressure can decrease selectivity of the membrane for gas permeation. On the other hand, reverse osmosis membranes have shown that salt permeation is sensitive to, among other variables, water content in the polymer and a fundamental attribute in ionic diffusion is the effective size of hydrated ions. In addition, external electron sources—e.g., cathodic protection potentials for pipeline structures—can alter the kinetics of this transport as the tendency of ions to dissociate increases due to electrostatic forces. Focusing primarily on epoxy-based powder coatings, this review demonstrates that service parameters such as humidity, temperature, and concentration of aggressive species can dynamically develop different transport mechanisms, each at the expense of others. Although multilayered coating systems decrease moisture ingress and the consequences of environmental exposure, this survey shows that demands for extreme operating conditions can pose new challenges for coating materials and sparse data on transport properties would limit analysis of the remaining life of the system. This knowledge gap impedes the prediction of the likelihood of coating and, consequently, infrastructure failures.

Water transport through epoxy-based powder pipeline coatings

“…This may be due to different FBE formulations, or it may reflect these authors applying an imperfect sealing system in their cup method (i.e., using "water-proof epoxy resin" as a sealant). The data presented herein are in good agreement with results published in other literature for epoxy materials [5], [47]. Our study indicates that although measuring water vapor transmission rate and permeability at high temperatures can significantly vary with water activity, cup methods, if carefully implemented, can generate data with excellent confidence.…”

“…Upon further temperature increase (above 70°C) in our experimental design, the FBE network was plasticized by vapor permeant and concentration (or pressure) of the permeant became a determining factor in the barrier performance of the coating. In other words, although the clustering effect was likely to decrease permeability of the coating membrane, and therefore increase the selectivity over other permeants, the role of a permeant concentration is expected to become more pronounced for the hydrated polymer at elevated temperatures [47]. The occurrence of minimum permeability at ~70°C is consistent with the observations of other researchers in glassy polymeric membranes [41].…”

“…Since water has a smaller kinetic diameter compared to other penetrants, it plays a key role in the barrier performance (i.e., selectivity or separation factor) of the coating. Thus, permeability data presented in this study enable quantitative analysis of the barrier performance of FBE and HPPC coatings [47].…”

“…Since solubility of water molecules is the main driver of vapor permeability in the FBE structure [47], an additional moisture resistant layer can mitigate detrimental consequences of interactions between water and the FBE. Results from Table 2 indicate that water vapor permeance can be reduced up to an order of magnitude via the multilayering approach.…”

Water transport through epoxy-based powder pipeline coatings

“…This may be due to different FBE formulations, or it may reflect these authors applying an imperfect sealing system in their cup method (i.e., using "water-proof epoxy resin" as a sealant). The data presented herein are in good agreement with results published in other literature for epoxy materials [5], [50]. Our study indicates that although measuring water vapor transmission rate and permeability at high temperatures can significantly vary with water activity, cup methods, if carefully implemented, can generate data with excellent confidence.…”