If amine blush occurs on your epoxy coating, it is very important that you correctly identify and remove it prior to recoating. Coating over amine blush has been the cause of countless and very expensive coating failures for many years.
Epoxy resin based products are generally cured with an amine functional co-reactant which cross-links with the epoxy groups to form a solid material that may function as a chemical resistant coating or lining. After the reacting mixture has been applied and before it is fully hardened (reacted), there is a potential of some of the amine hardener to react with moisture and carbon dioxide in the surrounding air. This reaction can produce a thin layer of an amine carbonate on the surface of the material. This phenomenon is referred to as an amine blush, because it usually imparts a cloudy or greasy appearance to the surface of the material. This tendency is greater at low temperatures (<60ºF) and high humidity or moist conditions. The likelihood and extent of this film formation varies greatly from one epoxy formulation to another.
Some systems do not exhibit this surface film except in very cool damp conditions, and then only to a minor degree. Other systems are very likely to develop a blush, which can be very heavy in cool, damp environments.
Amine epoxies are the most likely to blush, and great care should be taken when using amine epoxies in damp or cool conditions.
Epoxy-amine adduct curing agents have been designed to have a reduced tendency to blush. Epoxy-amine adducts are reaction products of liquid epoxy resin with an excess of primary amine. Although epoxy-amine adducts still contain a large excess of free amine, they are less hygroscopic and have a lower vapor pressure compared to the neat amines. Epoxy-amine adducts are less sensitive to blush formation and, as a result, are better suited for use in high humidity or low temperature conditions.
There are many detrimental effects of amine blush. If left uncoated, the surface will feel tacky or greasy, and it will be difficult to keep clean. Airborne contaminants may stick to the surface. The coating will have a reduced gloss and it is likely to amber rapidly. If the surface is coated prior to removing the amine blush, the adhesion of the next coat will be greatly reduced. The resulting adhesion would be similar to what you would expect if you were to paint a candle or a bar of soap.
Amine blush is not usually visually detectable, so it is often overcoated because the painters are not aware that it has developed. Fortunately, it does have a waxy, greasy, or oily feel. If you are using an amine epoxy and it has cured in humid or cool conditions, it is very important to perform a fingertip test for amine blush at a bare minimum. I prefer to spread out my fingers on the coating, like a spider clinging to the surface. As you pull your fingers together, you can usually feel the presence of amine blush if it exists. Amine blush has a high pH, but this does not necessarily indicate the presence of amine blush because the high pH may have been caused by other factors. A more definitive method involves the use of amine blush test kits, such as this one by Elcometer: http://www.elcometerusa.com/Elcometer-139-Amine-Blush-Swab-Test-Kit_2.html Using this test to compare the surface to a clean control surface will be very useful to determine the existence of the problem.
Amine blush is easily removed by washing with soapy water. Dawn dishwashing soap works well. Care should be exercised that the process washes away the blush and does not merely wipe it around on the surface. A sufficient amount of rinse water will ensure that the amine blush is removed from the surface. Care should be taken to flush and remove water from ponding areas to avoid accumulation of contaminants. Following the cleaning process, test again to ensure that the blush has been successfully removed on the entire surface to be recoated.
The easiest way to prevent amine blush is to avoid using epoxies in cool, damp, or humid conditions. If the environment is expected to be cool, damp, or humid during the cure cycle, consider delaying the application or changing the environment by heating or dehumidification. If you cannot delay the application or control the environmental conditions, contact me firstname.lastname@example.org I’ll help you choose a coating that will be most suitable to the expected environmental conditions.