Welcome back to our third in a series of posts about polymers and the characteristics of the various formulas and types that are utilized for protective coatings/linings, corrosion barriers and for structural renewal. We hope these will be helpful in developing a better understanding of the subtle yet very important differences in these products and their applications in infrastructure protection, rehabillitation and asset life extension.
Polymers Part III: Epoxy Coatings
As with polyurethanes and polyureas, epoxies are two-component systems using a resin and a hardener. In the water and wastewater management industries, epoxy coatings are primarily used for corrosion protection, since the hardened shell is impervious and non-reactive to most chemicals. Though, like other polymers, epoxy stands up well to a very acid environment, it’s particularly effective in areas where it will receive high exposure to alkaline products.
Epoxy is generally either sprayed, brushed or troweled onto the surface needing protection. The most common types of epoxies are Bisphenol A or F, and Novolac. Where polyureas and polyurethanes have a spot cure time measured in seconds, epoxy takes anywhere from 2 to 24 hours to fully cure.
Depending on the particular product, most epoxies are not what’s considered “high build.” A single layer application can’t go over 100 mils or a tenth of an inch, due to the slower cure rates and a tendency to form pinholes on improperly prepared substrates. Thicker coatings must be applied in layers. However, there are some new, high-build epoxies that have exhibited some success with single applications of greater thickness.
Corrosion millage thickness starts at 125 mils, if issues such as delamination caused by outgassing through pinholes are to be avoided, so this type of application will likely require more than one layer.
Epoxy forms a brittle, rigid surface that allows for very little tensile elongation, so it’s not a good candidate for areas of high vibration, or where there’s a high likelihood of any appreciable movement in the substrate. Its raw components and application labor are more expensive, so this is not typically a budget solution. However, it is nearly unsurpassed at what it does best: corrosion protection.