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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

Instrument Sales
1-800-KTA-GAGE
Lead Hotline
1-800-LEAD-OFF

The Case of theTortoise or The Hare...The "Rapid Recoat System" With a Slow Recoat Time!

By Rick Huntley - Senior Coatings Consultant, KTA-Tator, Inc.

Several years ago, a factory in the western United Stated was installing several thousand feet of pipe to carry hydrogen. Similar water piping was also installed. Before shipment to the facility, sections of the pipe were transported to a shop painting facility for surface preparation and painting. The pipes were prepared and painted in a building with one side open. The temperature of the air in the facility was approximately the same as the outside air. Painting took place when the ambient air temperature was between freezing and 55°F.

The piping was blast cleaned in accordance with SSPC-SP10, "Near-White Blast." The coating system applied to the piping consisted of one coat of a polyamide epoxy primer, one coat of a polyamide epoxy intermediate coat, and a polyurethane topcoat. The coatings were manufactured by a local paint company. The epoxy coats were recommended to be applied at 4.0 mils each; the polyurethane topcoat was recommended at 2.0-3.0 mils. The total dry film thickness for the system was recommended at 10.0 mils minimum. The manufacturer indicated that the recoat time for both the epoxy primer and intermediate coats is one to two hours after application at 70-80°F.

After application of the coating system, the pipes were transported to the facility for installation. The pipes received significant mechanical damage during transport. There were circular patterns of delamination around the circumference of the pipes and many parallel scrapes along the pipe. The painting subcontractor who was contracted to touch-up the pipe complained that the amount of mechanical damage was excessive. KTA-Tator, Inc. was contracted to independently investigate the extent of damage, determine the cause, and to make recommendations for repair.

The Field Investigation

A KTA consultant visited the facility and examined the subject piping. The piping was found to have significant mechanical damage, and the damage was located on all sections of the installed piping. There was no correlation between the amount of mechanical damage found and the location of the installation. While most of the mechanical damage appeared in a ring pattern around the pipe where it had been transported with nylon straps, there were also a great number of parallel scrapes. Generally, the damage affected the topcoat and part of the intermediate coat, but occasionally the damaged areas revealed removal down to the primer. It was estimated that 25-30% of the surface area required touch-up because of the abrasion damage. Normally, 5% or less of the surface should require touch-up due to erection damage.

The condition of the substrate was assessed by scraping away the coating using a utility knife. Upon examination, the steel had been appropriately blast cleaned to an anchor pattern of 1.0-2.0 mils and was free of any visible rust in both damaged and undamaged areas.

The dry film thickness of the coating system ranged from 8.0-18.8 mils, with an average of 13.0 mils. Although the dry film thickness readings varied considerably, there was no pronounced correlation between coating thickness and the amount of mechanical damage.

The adhesion of the coating system was evaluated by subjectively probing at the paint using a utility knife. The adhesion was fair; however, it was noted that the primer and intermediate coats were unusually soft for a catalyzed epoxy that had been permitted to cure for several months. The topcoat could be easily removed by scraping using the utility knife; the underlying primer and intermediate coats could be easily removed by lightly scraping at the surface. The hardness of the paint system varied considerably, but was poor in over 90% of the locations tested. Finally, in all locations evaluated, a strong solvent odor was present when the polyurethane topcoat was removed.

Laboratory Analysis

Samples of the epoxy primer and intermediate coat, and polyurethane topcoat were removed from the pipe and taken tot he KTA laboratory for analysis. The laboratory investigation involved analyzing the primer, intermediate coat, and topcoat using fourier transform infrared spectroscopy (FT-IR), applying wet samples of the coatings to test panels to determine thinning and recoat characteristics, and performing gas chromatography. FT-IR spectroscopy revealed that the epoxy primer, the epoxy intermediate coat, and the urethane topcoat were properly mixed. The spectra closely matched the spectra of KTA-applied control samples of the specified coatings. Accordingly, it was concluded that the failure was not a result of product substitution.

The physical testing program consisted of application of the coating system to 4" x 6" hot rolled carbon steel panels previously blast cleaned in accordance with SSPC-SP5 "White Metal Blast." Four panels were primed with unthinned epoxy primer; another four panels were primed using the same epoxy primer thinned 10%. The epoxy primer was applied at a wet film thickness of 6.0-7.0 mils, as recommended by the manufacturer. Some of the panels were permitted a one-hour cure time prior to being coated with the epoxy intermediate coat, others were permitted a 24-hour cure time. As with the primer, the epoxy intermediate coat was applied both thinned and unthinned, at approximately 6.0-7.0 mils wet film thickness. Once again, some of the panels were permitted to cure one hour, others were permitted to cure for 24 hours prior to the application of the urethane topcoat.

After application and proper curing of the polyurethane topcoat, the panels were evaluated for adhesion and hardness. Adhesion was evaluated in accordance with ASTM D-3359, Method B. This test involves making a series of parallel knife cuts, followed by a second series of cuts perpendicular to the first, creating a grid pattern, then applying a special adhesive tape to the grid, rapidly removing the tape, and assessing the amount of coating detachment. All panels exhibited excellent adhesion characteristics by this method, with a rating of 5B (no loss of adhesion).

In addition to performing coating adhesion, one panel from each set was also evaluated for pencil hardness (ASTM D-3363). Briefly, this test involves using a series of pencils having leads of various hardnesses, and determining which pencil will penetrate the film, and which will not. The pencils are rated from softest to hardest as follows: 6B-5B-4B-3B-2B-B-HB-F-H-2H-3H-4H-5H-6H. Pencil hardness was evaluated 1 1/2 weeks and one month after the panels were coated. The results are shown in the following table:

As the above results indicate, the systems that were allowed to cure for 24 hours were harder than the systems that were allowed to cure for one hour. All systems became harder over time.

At this point in the failure investigation, additional analytical testing was conducted. Gas chromatography was performed to determine the type of solvent retained in the coatings removed from the piping. The technique involved placing known weights of paint chips into closed septum vials, and partially immersing the sealed vials in an oil bath maintained at approximately 110°C (230°F). A gas-tight syringe was then used to sample the volatilized solvent within the vial, which was then injected directly into the gas chromatograph. The chromatograms revealed that no unauthorized solvents were used to thin the paint.

Several of the test panels prepared by KTA retained so much solvent that they emitted a strong odor. Accordingly, paint chips from these panels were weighed, then placed in an oven at approximately 150°F for five days. At the end of this time, the chips were reweighed, and the percent weight loss due to solvent evaporation was calculated. The results are shown in the following table:

The above results indicate that the more retained solvent the softer the coating, and that the combination of thinning and short recoat times cause greater solvent retention and softness.

The Cause of Failure

The results of the field investigation, the laboratory analysis, and the physical testing revealed that the extensive mechanical damage to the coating was a result of solvent entrapment which occurred because of the insufficient amount of time that was permitted prior to recoating the epoxy primer and intermediate coats. The epoxy primer and intermediate coats were recoated before sufficient solvent had evaporated to allow for proper drying of the coating. Solvent entrapment resulted which in turn, caused the paint to be soft. The manufacturer's product data sheet for the primer and intermediate coats stated recoat times of one to two hours at 70-80°F. However, gas chromatography and infrared analysis revealed that the specified epoxy product was similar to other epoxy coatings that have recoat times between 6 and 24 hours.

Both the epoxy and the polyurethane coatings used have good solvent resistance. When epoxies are recoated before sufficient amounts of solvent have evaporated, subsequent coats of an epoxy and polyurethane entrap solvents in the underlying coat. The entrapped solvent is very slow to migrate through the solvent resistant materials. The paint will cure, however the solvents act as a plasticizer leaving the paint extremely soft and lacking in cohesive strength. The sheer applied during normal transport of the paint pieces caused the solvent-rich coating to rupture, and delamination resulted.

Analysis of KTA-applied coatings revealed a significant amount of retained solvent in the paint, as indicated by large peaks on the gas chromatogram. If adequate recoat times are permitted during painting operations, only a slight amount of solvent would be detected in the paint after three months.

The physical-testing program was initiated to determine if the manufacturer's procedure for coating application was sufficient to ensure proper solvent evaporation before recoating. The testing revealed that when the primer and intermediate coats were recoated after one hour, (permitted by the manufacturer's data sheet), the paint retained considerable solvent and remained soft. The testing also indicated that if the system is allowed to cure for 24 hours before recoating, the coating system has adequate hardness, independent of whether or not thinner was added. While some solvent was detected on the samples that had a 24-hour cure time, but the amount of solvent did not noticeably soften the paint. Thus, the laboratory analysis and physical testing illustrated that the manufacturer's recommended one-hour minimum recoat time for this system is not adequate to remove sufficient solvent to ensure proper drying of the coating. Contractors that are involved in shop painting operations often desire coatings that can be quickly recoated. Because of this, coatings with fast recoat times are often more marketable than coatings with longer recoat requirements. In this case, the coating manufacturer recommended a rapid recoat time for a coating that traditionally has a relatively long recoat requirement (6-24 hours). While it could not be determined at which temperature the applicator applied the material, it was determined via laboratory testing that even if the coating was applied using recoat times and temperatures recommended by the manufacturer, the coating was still prone to this type of failure.

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