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KTA-TATOR, INC.
Corporate Headquarters
115 Technology Drive
Pittsburgh, PA 15275
Phone: 412.788.1300
Fax: 412.788.1306
E-mail: info@kta.com

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The Case of the Blistering of a Vinyl Swimming Pool Lining

By Valerie Sherbondy

A twenty year old municipal swimming pool, made of aluminum, was contracted to be repainted. The old coating, which was flaking badly from the aluminum, was removed by blast cleaning to SSPC-SP10, "Near-White Blast", and the aluminum was then cleaned. While the painting contractor was setting up equipment and materials were being brought in for the painting, wet spots were noticed on the lower level of the pool. The maintenance personnel discovered that a water main had burst approximately 30 to 40 feet from the pool, indicating that the wet spots on the lower level of the pool had resulted from pinholes in the aluminum plates. There was no water entry on the upper half of the pool, which was surrounded by a room containing piping and pump equipment.

The municipality decided that the pinholes required welding and brought in a welder to make repairs. After the repairs were made, the pool was again cleaned. The painting contractor then applied a vinyl primer, followed approximately four hours later by a second coat, and then again several hours later by a third coat of vinyl. The coating application was conducted in early May.

Additional repair work was also scheduled during this time. During the course of this work, the paint on the bottom of the pool had become discolored. The pool was filled and put into service for the summer, with the intention that it would be repainted in the fall. When the pool was emptied after Labor Day to be repainted, blisters were seen on all surfaces, including those above ground. When examined by maintenance personnel, the blisters were found to be liquid filled, and went down to the aluminum substrate. At this point, KTA-Tator, Inc. was contracted to investigate the problem.

Samples of the coating from the blister areas, samples of the blister liquid, samples of the pool water, and samples of the specified paint were sent to the KTA laboratory. The laboratory investigation consisted of visual and microscopic examination, infrared spectroscopy, elemental analysis, gas chromatography, and coating application to test panels.

Microscopic and visual examinations are quick methods for determining if the paint was applied over contaminants, such as dirt or oil, the dry film thickness of the applied coatings, or any other irregularities in the paint system which may determine the cause for the paint failure. The visual and microscopic examination revealed that three coats of paint had been applied, and all were within the recommended thickness ranges. There were no signs of pinholes or large voids through the coating. However, there was a white material on the primer side of the paint chips. When viewed microscopically, the white material appeared both crystalline and powdery. This material could easily be scraped from the primer, as opposed to being embedded in it, which indicated that it was not present when the paint was applied.

The white material was then analyzed by infrared spectroscopy. Infrared spectroscopic analysis is a technique where a beam of infrared light is passed through the sample. The amount of energy absorbed by the sample at different frequencies of light is then plotted, producing a spectrum. The different paint resins and pigments produce characteristic spectra which can be used to identify the paint, to determine if the specified paint was used, and to determine if the paint has degraded. This technique may also be used to identify certain contaminants. Infrared spectroscopic analysis revealed that the white material was inorganic and also that the coatings used were specified vinyls.

Since infrared spectroscopy was inconclusive in identifying the white material on the backside of failing coating, elemental analysis was performed to more completely characterize the material. For this technique, the sample was bombarded with an electron beam which causes the sample to give off x-rays. Each element emits x-rays of characteristic energies which can then be identified. The elemental analysis revealed that the white material was an aluminum corrosion product.

Since solvent entrapments is a well known cause of blistering, and since the three coats of vinyl were applied with minimal time between coats, the blister liquid was tested by gas chromatography for the presence of solvents. Gas chromatography is performed by injecting a gas or liquid sample into a heated column. The temperature of the instrument would cause the liquid sample to vaporize and become a gas. The sample is then carried through a packing material which separates the components of the sample. Different solvents produce peaks at characteristic times. The solvents of the paint or thinner may then be identified. This testing may also identify unauthorized thinners. This analysis failed to detect any solvents in the blister liquid.

At this point, two common causes of blistering could be ruled out. The first cause that had been ruled out was ground water. Since the blistering had occurred over the entire surface, including that above grade, it was not likely that ground water entry had caused the blistering, even though small holes had previously been found in the aluminum. The second cause of blistering, solvent entrapment, was also ruled out. This left one other likely cause for the blistering, salt (chloride) contamination.

Samples of the pool water and blister liquid were tested sing Quantab test strips. Quantabs, manufactured by Environmental Test Systems, Inc., are test strips which detect chloride. The results revealed that the blister liquid contained five times as much chloride as the pool water (500 part per million versus 100 part per million).

The concentration of salts on the metal surface would create an osmotic cell. An osmotic cell is created when there is a higher concentration of water soluble material, such as chlorides or other salts, on one side of the coating than on the other. Water will be forced through the coating by osmotic pressure in order to dilute the salt concentration behind the coating, resulting in blistering.

This phenomenon was duplicated in the laboratory. Four aluminum panels were contaminated with chlorides and painted with the three coat vinyl system, along with four clean panels. After drying, all eight panels were immersed in tap water. After two weeks, all of the contaminated panels had blisters (4MD or larger, ASTM-D 714), while the uncontaminated panels showed no signs of blistering.

After determining that chloride contamination had caused the blistering, the cleaning procedures of the contractor were reviewed. The specification stated that the walls of the pool were to be pressure washed prior to paint application. When the contractor was questioned about the cleaning practices used prior to painting the pool, it was revealed that the surface had been wiped down with solvent, rather than pressure washed with water. A solvent wipe down is commonly used to remove grease or oil from the surface. It was thought by the contractor that the blast cleaning and the solvent wipe would clean the aluminum surface and leave it well prepared for paint application. However, while the blast cleaning might have removed some of the chlorides, it would certainly not remove all of the chlorides, and the solvent wipe would simply serve to spread the chlorides on the surface. Water washing, as originally specified, would be needed to thoroughly remove the chlorides from the surface. Had this been done, it is entirely likely that the failure could have been avoided.

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