California Coating Expert Performs Failure Analysis of Epoxy Paint

A California coating expert was confronted with a municipal water tank with a rubbery epoxy coating. The complaint was that the epoxy paint was not properly mixed, and it had not cured properly. During the coating failure analysis, the study showed that the epoxy paint was aliphatic amine cured. In addition, the tank was painted in the winter, and it cured at 50°F. Prior to filling with water, the tank interior was given a final inspection. During the inspection the paint layer was found to be rubbery on the sunny side of the tank. Because epoxy is thought to be "cross linked" all were surprised that the paint was rubbery. and they were concerned that the paint adhesion had been compromised. Adhesion tests were positive. The fact is many epoxies 'soften' above their initial cure temperature. In this case, the epoxy paint had cured at or below 50°; thus, on a warm day (T> 70°F) the epoxy will initially be soft. However, over time, the paint will "re-cure" and harden at the higher temperature. The failure analysis demonstrated that there was no product defect.

Glass Transition Temperature of Epoxy Paints

California Materials Expert Discusses The Temperature Performance of Two Part Epoxy Coating:
Most users think that, because epoxies are listed as thermoset plastics, epoxies do not soften at elevated temperatures. Actually, after initial hardening epoxies demonstrate softening at temperatures above the initial cure temperature. This was the case for epoxy paint used to coat the inside of sanitary water holding tanks. The tanks were painted in winter; so, the initial cure was at approximately 55° F. When the weather warmed the surface of the tank reached 115°F, and the epoxy paint was found to be soft and rubbery.However, over time the epoxy paint became hard at the elevated temperature as the higher temperature became the effective curing temperature. The common view is that epoxies are thermoset plastics; however, they can soften under certain conditions