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The Case of the Fix That Was as Bad as the Problem
By Rick Huntley, Coatings Consultant, KTA-Tator, Inc.
In the early 1990s, a fire occurred in a water treatment plant in the Eastern United States. The fire occurred when welding operations ignited some plastic tube cylinders while repairs were being made to a process tank. A thick smoke engulfed the entire facility, necessitating a thorough cleaning, deodorizing, and repainting project
The water authority hired a general contractor who subcontracted the cleaning and repainting services. The specification prepared for the project called for interior block walls and steel roof trusses in the facility process room, pump room, office spaces, and bathrooms to be painted with two coats of a two-component epoxy polyamide coating. The coating had a recommended dry film thickness of 3 mils per coat.
When the facility was originally painted in the early 1980's, the process tanks were empty. During the repainting after the fire, the process tanks were in full operation. Since the facility was completely enclosed, the relative humidity was very high due to evaporation from the tanks.
Shortly after application of the white two-component epoxy paint, it was noticed that the surface of the paint began to discolor. After a short period of time, some of the surfaces turned to a relatively deep yellow. The painting subcontractor was requested to repaint the entire facility, including those areas that had not discolored. The subcontractor contacted KTA to determine the cause of the discoloring.
The facility was visited by KTA nine months after it had been repainted. By that time, there was various degrees of yellowing of the epoxy paint throughout the facility. On the west wall of the processing room, the yellowing was the most severe. Well water entered the room from beneath the severely yellowed wall. The yellowing appeared in a streaky pattern with the streaks aligned vertically. On the other walls, the yellowing occurred mostly on bottom 5 feet, although occasionally appeared at higher elevations. The yellowing was widespread, but relatively light on the roof trusses. In some areas, a white, non-yellowed paint was applied over a yellowed paint. Testing performed on this paint revealed that the adhesion of the paint applied over the yellowed paint was excellent. The adhesion was also found to be excellent in all other areas, including areas that had not yellowed and areas that had severely yellowed.
Since two-component epoxy coatings developed good solvent resistance upon curing, solvent sensitivity of the paint was assessed at the site by soaking a Q-tip with methyl ethyl ketone (MEK) and rubbing the surface of the coating 50 times. The solvent sensitivity was found to vary considerably, but generally was found to be much worse in areas that had yellowed and better in areas that had not yellowed.
Samples of paint from yellowing and non-yellowing areas were removed and sent to the KTA laboratory for analysis.
Paint chips that were removed from yellowed and non-yellowed areas were examined in the laboratory using Fourier Transform Infrared Spectroscopy. This method is capable of determining the mix ratio of the coating by comparing the unmixed components of the epoxy paint to a properly mixed sample of the paint. The samples from the field are then analyzed and compared with the control samples to determine the mix ratio.
The analysis found that the severely yellowed samples had a relatively large amount of polyamide resin as compared to the epoxy resin. The ratio of polyamide to epoxy was much higher than in a properly mixed sample. The non-yellowed samples had the proper ratio of polyamide to epoxy and appeared to have been properly mixed. All samples were found to be the specified two-component epoxy material.
The observations made during the field investigation and the laboratory analysis indicated that the cause of the discoloration was moisture condensation on the freshly applied paint. During the field investigation, the temperature inside the facility was estimated to be slightly less than 60°F, although the outside ambient temperature was in the mid 70s. The open water tanks kept the temperature in the facility relatively low and the humidity high. These conditions are favorable for moisture condensation.
Epoxy polyamide and epoxy amine paints have a tendency to blush in moist, cold conditions. The cold temperatures retard the curing of the coating, and condensing moisture is able to permeate the coating before it cures. Polyamide or amine is transferred to the surface along with the water upon evaporation, because both the polyamide and amine are slightly water soluble. Unfortunately, these materials are difficult to detect visually. They appear as a slick, clear surface contaminant, but upon exposure to carbon dioxide in the air, they turn yellow.
In this case, the yellowing only occurred near and above water tanks. The coating was applied to the entire facility, including office spaces, bathrooms, laboratory spaces, and the pump room, but no yellowing was found in the other areas. All other areas were operated at a higher temperature and likely had a much lower humidity.
The yellowing occurred somewhat randomly throughout the process tank room. Epoxies cure by chemical reaction, and the chemical reaction starts immediately after mixing.
During the chemical reaction, the polyamide materials that can cause yellowing react with the epoxy. The longer the polyamide is allowed to react with the epoxy in the can, the less the polyamide has a tendency to leach out when exposed to moisture. Thus, one difference in the severity of the yellowing may have been dependent upon the amount of time the coating was allowed to react in the can before application. Those paints applied shortly after mixing likely yellowed much more severely than those coatings applied much later after mixing.
The yellowing was also greatly affected by the mix ratio. The yellowed samples had an excess of polyamide in the mix, and this caused the paint to have a greater tendency to leach polyamide.
Fortunately, the adhesion of the coating was excellent in all areas where it was applied over the yellowed epoxy. Because the adhesion was found to be good in all areas, there was no need to remove the yellowed paint. The facility was repaired by simply solvent cleaning the walls and reapplying a properly mixed two-component epoxy coating. The process tanks were temporarily covered during the reapplication, and heaters were used to increase the temperature in the facility to prevent condensation.
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