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Carbamate formation in epoxy resin coatings

Unwanted white discoloration during curing

Fortunately, carbamate is not an overly stable chemical compound and can usually be removed completely with appropriate measures.

The occurrence of carbamate on EP coatings is primarily an optical defect

Almost every processor of epoxy resin-based floor coatings has experienced this undesirable phenomenon at first hand.

During the curing process, unattractive white discolorations appear in spots or even over large areas on an applied coating. The discolorations, which are visually reminiscent of water stains or lime deposits, but in contrast cannot simply be rinsed off, cause discontent among processors and builders, especially in the final sealing layer.

The resulting whitish substance is known by the term carbamate. In the cooler seasons of autumn and spring, when synthetic resin floor coatings are still being applied, for example in parking garages and underground garages, the risk of unwanted carbamate formation on construction sites is increased. An understanding of its causes and possible countermeasures is therefore particularly important.

Chemically, carbamate formation is due to the reactive components used in epoxy resin coatings.

The amine curing agents used cause the liquid resin component to crosslink, thus creating a rigid, three-dimensional network - the cured epoxy resin. In addition to this intentional reaction, however, the amines can react unintentionally with other substances present in the environment due to their high reactivity. Thus, amines can react with the participation of water and carbon dioxide to form carbamate - a salt that appears as a white, matte, sometimes slightly sticky layer.

The occurrence of carbamate on EP coatings is primarily an optical defect and is particularly disturbing when it occurs in the sealing layer, which is subject to high aesthetic demands.

However, large areas of carbamate can also cause problems in underlying layers, such as a primer. The reason for this is the unwanted reaction of the amines to a non-reactive intermediate layer. Consequently, reactive partners are missing to chemically bond with a subsequent layer and intercoat adhesion may be impaired. Resulting carbamate should thus be removed from all layers of a multilayer coating system for both visual and functional reasons.

Fortunately, carbamate is not an overly stable chemical compound and can usually be removed completely with appropriate measures.

Dilute organic acids are used to dissolve the salt. In most cases, the affected areas are treated with 10-20% citric acid.

In the case of strong carbamate formation, the somewhat unpleasant smelling acetic acid can be used alternatively or as a supplement in approx. 10% concentration. Both acids are worked into the contaminated surfaces using a scrubbing brush, sponge or cleaning machine and left on the surface for about 10-30 min. The surface is then rinsed with sufficient clear water. Both the concentration of the acids and the duration of the treatment should be selected according to the strength of the carbamate expression.

However, even if carbamate can be removed from a floor coating by comparatively simple means, its formation should of course be prevented at best

In the spring and autumn seasons, the climatic conditions during the application of epoxy resin coatings are often close to the dew point, which favours the formation of carbamate. The following preventive measures to avoid carbamate should therefore be considered in particular during these seasons:

Exact adherence to the mixing ratio of the EP products to avoid excess amine
Protection of the not yet fully cured surface from water exposure
Adherence to the manufacturer's specifications on ambient temperature and relative humidity during processing and curing, especially also when curing overnight
Avoidance of falling below the dew point (=condensation of humidity on the surface) during application and curing by means of air conditioning and/or scheduling of the construction site to suitable weather conditions
Use of EP products with reduced carbamate susceptibility

VIACOR epoxy products are tested during their development for critical processing conditions, such as very high humidity and early water exposure, and formulated using special raw materials that reduce the risk of carbamate formation. Provided that the instructions given on the technical data sheets are observed, the formation of carbamate during processing on a construction site can thus be virtually ruled out. Once fully cured, VIACOR EP coatings are of course completely resistant to discoloration caused by moisture.

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