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The Case of the Dissolving Railcar Paint
By Rick Huntley, KTA-Tator, Inc.
A railway transportation company that owned and operated a large number of railcars in numerous locations recently began to experience problems with the coatings on various railcars and locomotives. Most of the cars had been primed with a single-component, green-alkyd paint. On the exterior surfaces, the alkyd primer was topcoated with a solvent-borne two-component polyurethane coating. The polyurethane topcoat was applied in a variety of colors, depending on the location, but was always high gloss.
The railcars journeyed throughout the United States but maintenance on the cars was always performed in a major southwestern city. Maintenance consisted of routine washing of the locomotives and cars and performance of any necessary mechanical repairs. Washing of the railcars and locomotives was performed by spraying on a solution of a popular industrial cleaner. The cleaning solution was supplied to the company in a concentrated form and diluted in the rail yard before application. The recommended dilution ration was one part cleaning solution to 200 parts water.
A few years ago, workers in the rail yard began to notice problems with the paint. In numerous areas, there were circular areas of delamination of the topcoat. On some of the interior spaces, it was noticed that the primer was severely deteriorating. As a result, KTA-Tator was contracted to investigate the coating failures to determine the cause and to recommend methods to prevent further failures.
Field Investigation
At the time of the site visit, several locomotives and numerous railcars were available for inspection. All were coated on the exterior with very high gloss urethane paint. Although the paint on many of the railcars was several years old, there appeared to be very little loss of gloss. There was no evidence that the urethane topcoat had experienced ultraviolet degradation, chemical degradation or any other form of degradation.
There were numerous areas of delamination of the coating system. Many of the delaminations were circular and from one to three inches in diameter. There were also many blisters in the exterior coating. In all cases, a hole was present in the middle of the adherent blister cap. Occasionally, a green liquid was present in the blisters below the level of the hole.
The coating system applied to the cars was somewhat unusual. Typically, a two-component urethane is applied over other chemically cured coatings. In this case, it was applied over a single component alkyd coating. To determine whether this combination of coatings was causing adhesion problems, the adhesion was measure in several locations in accordance with ASTM D3359, Method A, "Measuring Adhesion by Tape Test." This method involves making two intersecting cuts through the coating to the substrate in an X pattern. A special adhesive tape is then attached to the X-cut area and sharply removed. The adhesion is rated according to the amount of the paint removed by the tape. In all areas not directly over a visible defect in the paint, the adhesion was found to be excellent.
The thickness of the coating was also measured in using a magnetic dry film thickness gage calibrated to National Institute of Standards and Technology plates. The thickness varied somewhat from 3.5 mils to 9.6 mils but there was no noticeable correlation between the amount of failure and the thickness of the coating.
The primer on the interior spaces was in considerably worse condition than the exterior coatings. In many cases, it appeared that the untopcoated primer dissolving. There were numerous vertical streaks in the coating of a green sticky material. The problem occurred mostly on the inside of door panels where the opposite side of the door panel faced the exterior.
Samples of the paint, the residue from the blisters and a sample of the diluted and undiluted cleaning solution were taken to the KTA-Tator laboratory for analysis.
Laboratory Analysis
Infrared spectroscopy was performed on samples of the alkyd primer taken both from an area of failure and from a non-failing area. The technique involved combining sample scrapings with potassium bromide powder, and forming a pellet under high pressure. The pellet was then placed in the optical path of the spectrophotometer, and a spectrum was obtained. The analysis revealed that the primer in both the failing and the non-failing area contained an alkyd resin. Unlike the sample taken from the non-failing area, the sample taken from the failing area absorbed infrared light at about 1850 cm-1, which is an indication of saponification. Saponification is the production of carboxyllic acid salts upon disruption of ester linkages, under alkaline conditions.
The diluted and the undiluted cleaning solution were also analyzed to determine the concentration of the diluted solution relative to the undiluted solution. A pre-weighed amount of the undiluted solution was placed in a glass dish and the water and any other solvent was allowed to evaporate. An identical procedure was performed on the diluted solution. The relative concentration of the diluted solution was determined by comparing the weights of the remaining non-volatile portion of the solution. It was determined that the cleaning solution had been diluted at a ratio of approximately 2 parts water to one part undiluted cleaning solution. The recommended ratio was 200 to 1.
The pH of the diluted cleaning solution was measured with a pH meter. The pH was 12.5.
Cause of the Failure
The field investigation and the laboratory analysis determined that the cause of the delamination of the coatings and of the deterioration of the primer was saponification of the primer due to exposure to the highly alkaline cleaning solution. The coating system used on the cars consisted of an air-curing alkyd primer topcoated with a two-component polyurethane coating. Regardless of the exposure, this system has inherent risks. Any time a chemically cured, solvent-resistant topcoat is applied over an oxygen-cured, solvent-sensitive coating, there is a risk of the solvent-resistant topcoat lifting the primer, or alternately, of the topcoat cracking. Adhesion tests performed on the exterior coating system indicated that the adhesion was satisfactory, and no cracking of the topcoat was found. Thus, incompatibility of the topcoat to the primer was ruled out as a contributing factor to the failure.
The pattern of failure provided the most significant clue as to the cause of the failure. The polyurethane topcoat had not noticeably lost gloss and was in good condition. Even in areas of blistering, the urethane film appeared to be in good condition, except for the blisters. The alkyd coating, conversely, was in poor condition. In untopcoated areas, the primer appeared to have partially dissolved. In many areas, the primer was tacky.
The blisters and the circular areas of delamination provided further evidence of the cause of failure. There was always a hole in the topcoat in the middle of the each blister. When a blister cap was removed, the green alkyd paint beneath was usually nearly totally destroyed and often only a green liquid remained on the bottom of the blister. Additionally, most of the primer was missing from the circular areas of delamination.
The pattern of failure indicated that the coating system had been exposed to something that had the ability to destroy the primer, but had little effect on the topcoat. Although urethane coatings have a superior resistance to a number of materials that will attack alkyd coatings, exposure to either a strong solvent or an alkaline solution is the most likely culprit. Solvents were unlikely to cause the untopcoated alkyd primer to remain permanently tacky, so exposure to an alkaline solution was the most likely cause of the problem. When an alkyd primer comes in contact with a alkaline material in the presence of water, the alkyd is attacked through a process known as saponification. The alkyd coating is converted into a water-soluble soap. The laboratory infrared analysis confirmed that the alkyd primer was indeed undergoing saponification.
The KTA laboratory investigated the cleaning solution as a possible source of the problem. The solution of diluted cleaning solution removed from the railyard had a pH of over 12 (highly alkaline). Since a 200 to 1 dilution a cleaning solution is unlikely to yield a solution with a pH over 12, is was suspected that the railyard employees were not sufficiently diluting the solution before spraying on the railcars. The laboratory analysis confirmed that the dilution ratio was 2 to 1 instead of the recommended 200 to 1.
As a result, the highly alkaline, concentrated cleaning solution attacked the alkyd primer anywhere there was a hole in the urethane topcoat. The attack continued until the coating blistered and in some cases delamination occurred. Where no topcoat was present, the alkaline solution slowly deteriorated the primer until eventually much of it washed away.
It was recommended to the railyard that the cleaning solution be properly diluted before application to the railcars to prevent the rapid deterioration of the primer. It was also suggested that a non-alkaline cleaner would be less destructive to this type of coating system and that use of non-alkaline solutions should be investigated.
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