The Lincoln Memorial Reflecting Pool has had a rough summer. The “American flag blue” the renovations were intended to bring out was first thwarted by algal blooms. After being treated with biocides, the pool was briefly restored before sections of the lining began blistering, peeling, and separating from the concrete.
Accusations of vandalism followed and are still under review, but the coating itself was definitely causing problems. The New York Times reports that documents submitted by Atlantic Industrial Coatings, the contractor for the project, suggest that the company has been working through several possible explanations for the failure, including too little primer, excess heat between coating layers, and even workers’ sweat.
Without examining the failed coating and records on its application, polymer chemists can’t say precisely what went wrong. But the explanations Atlantic has offered are chemically plausible, they say, and the underlying causes—primer coverage, heat, layer compatibility, and moisture—are familiar ones that experienced coatings manufacturers and applicators should be prepared to control.
The pool’s lining is a stack of polymers. National Park Service contract documents specify four Rhino Linings products that form an “integrated, monolithic waterproofing assembly.” The preparation starts with a specialized polyurea to fill all joints, followed by another polyurea tier to cap them. An epoxy primer is applied to the concrete base and Pipeliner 5000, a polyurea coating roughly 2.54 mm thick, tops everything off.
The epoxy primer is key to getting all those materials to stick together, and Atlantic later said workers had failed to apply it in sufficient amounts in some areas. “If the primer’s not there, you’re expecting the polyurea to adhere to the concrete, which is questionable,” says Dean Webster, a coatings chemist at North Dakota State University.
Polyurea is a tough, flexible polymer coating formed by reacting isocyanates with amines. The layers can also be formulated so that reactive groups form covalent connections across their interfaces, says John Texter, a polymer scientist at Eastern Michigan University.
The contractor reportedly concluded that two unspecified materials were incompatible and that the upper layer was hotter than the material underneath could withstand, which caused it to blister and come loose. When workers stopped applying the hotter material on top during repairs, blistering decreased but did not disappear.
Both the reaction that forms polyurea and the way it was applied to the bottom of the pool could have introduced unexpected heat.. “When an amine reacts with an isocyanate, it generates a lot of heat,” Texter says. How quickly and thickly a material is applied can affect how much heat builds up.
Webster reviewed data sheets for similar polyurea systems offered by Rhino Linings and notes that polyurea components are also applied hot to reduce viscosity. The recommended temperature of about 60-71 degrees Celsius alone shouldn’t degrade an epoxy, though it can soften it. But Webster says some of the epoxy’s additives or solvent components could be released with heat and get trapped between the layers, thus leading to blistering.
Excess water can also cause blistering and can come from several places. Atlantic reportedly suspects that sweat from a worker in protective gear had become trapped between freshly sprayed layers. Water can react with isocyanates and generate carbon dioxide gas—making occasional droplets a plausible source of sporadic blistering, Webster says.
Texter would also look beneath the coating at the concrete base of the pool. Moisture can remain in concrete or migrate through it from below. “The dryness and the source of water coming out of the concrete is the biggest worry,” he says. Ambient humidity is another variable that workers must account for.
None of these concerns are particularly unusual for this type of coating system. Before awarding the contract, the National Park Service wrote that installing the system requires “precise sequencing” and “temperature-controlled plural-component application.” The agency specifically warned that improper installation of any one of its materials could cause “delamination, blistering, discoloration, microbial growth, or premature failure.”
Texter describes epoxy and polyurea coatings on concrete as “a standard high-quality approach.” He adds that controlling the heat generated during application “is a valid concern, and that’s why an experienced contractor would know how to deal with that and control it.”
