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August 20 - August 27, 2016

What causes amine blush in epoxy topcoats?

Selected Answers

From Klaas de Wit of de Wit Consulting on January 8, 2017:
There is very good writing on this question from  M. Tuchscherer in 'Amine Blush Testing: Elusive Mystery or Good Old-Fashioned Organic Chemistry?' Paint & Coatings Industry (28) ’12, 5 May, p. 48-52, 54-56

Amine-cured epoxies are sensitive to amine blush. Blushing is caused by absorption of moisture and carbon dioxide from the atmosphere during the curing process. This can happen when the coating is subjected to a drop in temperature shortly after application. Carbon dioxide in the air along with any moisture that is present combines with the amine-curing agent and forms a carbamate on the surface of the coating. This carbamate is an oily-feeling, sticky liquid that cannot be coated over. It must be removed prior to overcoating. Blush is relatively easy to see with the unaided eye. View the surface under a good light, then rub a swab or gloved finger across it to disturb the oily layer enough to refract the light and the blush becomes visible. Sometimes, a greasy feel is evidence enough. There are a number of commercial methods to determine if blush is on the surface.

From Chuck Pease of MMI Tank on August 29, 2016:
To that end, with all of the possibilities of amine blush, why then wouldn't the engineer specify a moisture-cured urethane in lieu of an epoxy topcoat?

From john schultz of o-gee paint co on August 29, 2016:
We see that under certain conditions amine blush is elicited but I believe there are epoxies that are formulated in such a way to be more or less blush resistant. To that end, I have seen advertisements for marine epoxies that state they will not blush. On the other end I have seen TDS that state a particular epoxy should not be applied in conditions of ~>85% humidity. It seems then that the formulation of the coating has a lot to do with whether or not it will blush in a particular environment. If this is the case then what causes amine blush is unexpected environmental conditions after application. Either that or specification / use of an epoxy that is inappropriate for high humidity conditions.

From Warren Graves of Warren Graves Coating Consultant inc on August 29, 2016:
George Mays wrote a complete analysis of amine blushing in the November 2008 issue of JPCL. Mr. Mays explained that it is the reaction of carbon dioxide with the amine component of epoxies that forms amine carbamate. When there is enough moisture in the air, this reaction product further reacts to create amine blush. However, I can tell you from experience that there are times when you will not see the amine carbamate blush. When a tank interior is heated, such as in the winter in northern climates, in order to provide optimal application temperatures the humidity in the tank can be so low that the blushing will not take place. The carbamate exude remains on the surface as a thin, glossy oily layer. There simply is not enough moisture present to cause the blushing to occur.

From James Prevatt of SPEC-GUARD on August 26, 2016:
Coatings manufacturers typically make epoxy with a percent or two of excess catalyst by weight to assure full curing. Most amines have an affinity for water and take on either a whitish appearance or tacky feel, which must be removed prior to applying additional coats or the bond will be at risk. Soap and water wash with agitation or solvent wipe can address this issue. There are some cases where no appearance or tacky state can be observed. To be sure, use pH paper and if the result is alkaline, you know amine blush has occurred. Always remove the blush prior to taking the next step.

From Irvin Hosford of Tennant Company on August 25, 2016:
Amine blush usually occurs at high humidity under slow curing conditions (which happens to be at low temperatures in most instances). Condensation of water at the surface could promote it as well. Excess carbon dioxide from fuel combustion sources should be avoided when the epoxy is curing. Blushing can be prevented by using low-blush amines, the proper surface additive package, keeping the stoichiometric ratio of amine slightly lower than the epoxide content, and increasing cure speed without using faster curing amines, but rather catalytic components like benzyl alcohol and/or tertiary amines.

From Eric Piotrowski of SSPC: The Society for Protective Coatings on August 25, 2016:
High humidity and/or cool temperatures during application or curing will cause the blush condition.

From shahram akbartbar of mapna boiler on August 24, 2016:
Amine blush can cause surface tackiness or greasiness, incomplete cure, poor adhesion, poor adhesion on over coating, coating discoloration over time and poor gloss retention.

From David Zuskin of Indepedent on August 23, 2016:
Cool, damp and still conditions can cause amine blush of epoxy topcoats.

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Tagged categories: Asia Pacific; EMEA (Europe, Middle East and Africa); Epoxy; Epoxy amine; Latin America; North America; Topcoats

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