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|Federal Highway Administration > Publications > Public Roads > Vol. 62· No. 3 > Laboratory Testing of the Performance of Moisture-Cured Urethanes on New Steel|
Laboratory Testing of the Performance of Moisture-Cured Urethanes on New Steel
by Shuang-Ling Chong and Yuan Yao
Millions of dollars are spent each year on the maintenance of painted steel bridges and the repair of old bridge coatings in this nation. To reduce such costs, the use of climate-tolerant, durable coatings has become essential to ensure extended painting seasons and coating lives. In the last decade, the Federal Highway Administration (FHWA) has been actively involved in the study of such coatings and has identified several durable products that are now being used widely in the country. These coatings comply with the regulations established by the U.S. Environmental Protection Agency, which continues to lower the allowable amount of volatile organic compounds (VOC) emitted from industrial and maintenance coatings. This article reports the performance of moisture-cured urethanes on new steel surfaces.
Moisture-cured (MC) urethanes were designed to protect steel bridges from corrosion in high-humidity climates, especially in coastal areas. Urethane formulations originated in Germany and were brought to this country in the 1980s.
These coating systems contain micaceous iron oxide (MIO) and are known for their adhesion to steel and good abrasion resistance. These characteristics are reportedly attributed to the lamellar crystalline structure that shields ultraviolet (UV) light. This coating material consists of a single resin component that forms cross-linking polymers through a reaction with moisture from the air.
MC urethanes offer convenient application properties. They come in single packages and have longer shelf and pot lives than most other coatings. In addition, these fast top-coaters have low-temperature and high-humidity application properties. MC urethanes have been applied to bridges with blast-cleaned (SSPC SP-10 or SP-6) and power-cleaned steel surfaces (SSPC SP-3) and have shown good performance to date. Many states have used MC urethane coatings to protect steel bridges, some of which still remain in extremely good condition with less than 1 percent failure after eight years of service. Schwindt discusses some of the technical, practical, and economic aspects of this type of coating.
In a previous FHWA study, zinc-rich MC urethanes from a particular supplier were found to perform extremely well. After a 3,000-hour laboratory test and 28 months of outdoor exposure, no signs of failure were found either on the steel surfaces or at the scribes. However, there has been some inconsistency in the field performance of MC urethanes manufactured by different companies. To verify and ensure the reported performance of MC urethane formulations, it is necessary to study and compare various products and to establish critical parameters affecting their performance.
Three urethane systems (A, B, and C) were used for testing. (See table 1) Each system consisted of three coats with a total film thickness of 200 to 225 microns (8 to 9 mils). The VOC content of the coating material was equal to or less than 340 grams per liter (2.8 pounds per gallon). The coatings were applied on two types of steel surfaces: SSPC SP-10 and chloride-doped (20 micrograms per square centimeter [mg/cm2]) SSPC SP-10. In addition, a 5.1-centimeter (2-inch) scribe, a cut through to the steel surface, was made diagonally on all the coated panels to study the extent of blister and rust creepage that could possibly develop along it. The scribe breeches the coating to study the potential performance of the coating system at the site of any holidays (defects in the coating).
Duration: 68 hours
Temperature: -23°C (-10°F)
Duration: 216 hours (9 days)
Test cycle: 4-hour UV/4-hour
UV lamp: UVA-340
UV temperature: 60°C (140°F)
Condensation temperature: 40°C (104°F)
2. ProhesionT (cyclic salt-fog): ASTM G85
Duration: 216 hours (9 days)
Test cycle: 1-hour wet/1-hour dry
Wet cycle: A Harrison Mixture of 0.35% ammonium sulfate and 0.05% sodium chloride was used.
Fog was introduced at ambient temperature.
Dry cycle: Air was preheated to 35°C (95°F) and then purged to the test chamber
The volatile and pigment content of all paint material was determined using standard American Society for Testing and Materials (ASTM) methods. The elemental contents of the pigments were determined by a combined scanning electron microscopy /energy-dispersive x-ray spectrometry technique (SEM/EDS). The metallic zinc, as well as total zinc present in the isolated primer pigments, was quantified by titration methods. The particle size distribution of the pigments was obtained with a particle-size analyzer that measured the reduction in light intensity after the light passed through a particle-suspended solution. Other physical properties, such as adhesion strength of the coating systems and hardness of the topcoats, were also measured.
A cyclic laboratory test method, the Freeze/UV-condensation/ProhesionÔ test, was employed to evaluate the performance of the three MC urethane coating systems. All tests were conducted for a total duration of 4,000 hours (eight 500-hour test cycles). The panels were examined for surface failures (e.g., blistering and rusting) and were measured for scribe creepage at 500-hour intervals.
Results and Discussion
The chemical composition of the three MC urethanes varied in terms of formulation. The difference in type of binder (isocyanate) is indicated by the Fourier Transform Infrared (FTIR) analysis of the MC urethanes. Table 3 shows the relative amount of aromatics to aliphatics of isocyanate that can be estimated from the ratio of peak intensity at a wave number of 3100 cm-1 to that of 3000 to 2800 cm-1. The primer and intermediate coats of System A are most aromatic in nature, whereas those of System C are generally more aliphatic. However, all topcoats are virtually aliphatic in character and provide high UV resistance and hardness.
Table 4 shows the chemical composition of the zinc-rich primers in terms of volatile, solid, pigment, and binder content. This composition varied slightly among the three systems. System A was found to contain the highest amount of binder (presumably all isocyanate) and the least amount of pigment, System C had the lowest amount of binder and the largest amount of volatiles. Table 5 shows the metallic zinc and total zinc content of the three primers. Obviously, zinc is the primary ingredient in all the zinc-rich primers, but Primer C contained the highest amount of it. Other elements present included magnesium, aluminum, silicon, iron, and oxygen.
The particle-size distribution of pigments A, B, and C is shown in figure 1. It is arranged in increasing order (Pigment B < Pigment A < Pigment C), and the peaks occur at 5 micrometers (mm), 3 mm, and 10 mm, respectively. This distribution indicates that different grades of zinc particles were used in the three different primers. The smaller the particles, the tighter the packing of the coating film became. Proper gradation of pigment particles resulted in a less porous film, thereby decreasing permeability and enhancing barrier properties. In addition, tight packing of zinc particles increased conductivity, which in turn increased their cathodic protection to steel.
The pencil hardness (2H) of the topcoats and the adhesion strengths (10.5 megapascals [MPa] or 1,500 pounds per square inch) stayed fairly constant throughout the laboratory test. After performing the pull-off adhesion test, the coating failure modes were cohesive failure of either intermediate coats or topcoats. There was no adhesive failure between primer and substrate, thus demonstrating that the adhesion strength between all primers and steel was higher than 10.5 MPa. This value is large enough to reduce underfilm corrosion.
Throughout the 4,000-hour laboratory test period, no surface failures were observed on any of the MC urethane test panels. This proves that MC urethanes are excellent barrier coatings. However, the three systems developed rust creepage at the scribe approximately 1,500 hours after the test started. By the end of the test, all systems developed blistering and formed a continuous bulk surface along the scribe. Figure 2 shows the creepage growth for the blast-cleaned SSPC SP-10 surface as a function of test time. (Creepage values are an average of the five test panels.) It appears that all systems exhibited slightly different amounts of creepage at each time interval, but the rates of formation did not differ much. The amount of creepage on the chloride-free SP-10 surface increased in the order of B<C<A, but the difference was very small. Creepage on the chloride-doped SP-10 surface, on the other hand, increased in the order of B<A<C, as shown in figure 3. Creepage not only developed earlier in the test on this kind of surface (~1,000 hours), but also the amount of creepage was higher than that on the clean SP-10 surfaces.
Table 6 summarizes the overall failures that occurred by the end of the 4,000 hours. These results suggest that System B outperformed the other two systems at the scribe. It is speculated that the differences in scribe creepage might have been due to the difference in binder/pigment ratios. These ratios were calculated as 0.20, 0.15, and 0.11 (table 4) for products A, B, and C, respectively. The zinc-rich primer in System C had the lowest ratio, meaning that there was insufficient binder to fill the voids between the system's large pigment particles. As a result, System C exhibited the poorest performance at the scribe among the three chloride-doped systems. System C's failures included large rust-filled blisters (2D, 4D) and a 5.1-millimeter creepage that was twice as large as the one generated on the chloride-free SP-10 surface (2.5 mm). The effect of chloride on the performance of systems A and B was less severe than that on System C.
It is clear that the chloride-doped test panels developed more severe blistering and a larger scribe creepage than the chloride-free panels. Chloride contamination, even at the 20-mg/cm2 level, can have detrimental effects on coating performance. It has been reported that epoxy zinc-rich (organic zinc-rich) coatings can tolerate up to 30 mg/cm2 of chloride contamination without having any significant effect on performance. However, this work suggests that a 20-mg/cm2 level of chloride contamination is large enough to reduce the corrosion resistance of MC urethanes at the scribe on new steel even though primers contain more than 80 weight-percent of zinc.
1 ASTM D2369.
2 100 wt% - wt% Volatiles.
3 ASTM D2371.
4 100 wt% - wt% Volatiles - wt% Pigment.
1 ASTM D521.
2 Value on product data sheet.
Table 6 - Scribe Failure Results of MC Urethane Systems on SP-10 and Chloride-Doped SP-10 Surfaces After a 4,000-Hour Test
1 ASTM D714.
2 Chloride-doped surface
After 24 months of outdoor exposure to a marine environment, none of the three coating systems exhibited failures other than rust at the scribe. Although no data is available on field failure as of yet, a good correlation is expected to exist between laboratory and field test results.
Summary and Conclusions
The three MC urethane systems tested showed strong mechanical properties, such as adhesion strength of the whole coating system and hardness of the topcoats. The chemical composition of the three coating systems varied in terms of type and amount of binders and pigments.
In general, no surface failures were found on any of the test panels after the 4,000-hour accelerated laboratory test. This demonstrates that MC urethane systems are excellent barrier coatings for steel. However, the three systems applied on SSPC SP-10 steel surfaces developed different degrees of scribe creepage at the end of the cyclic laboratory test. Scribe creepage measurements are quite sensitive techniques used to differentiate coating performance prior to surface failure. As expected, the coating systems that were applied to the 20-mg/cm2 chloride-doped SSPC SP-10 steel surfaces developed more creepage than those on the chloride-free steel surfaces. The failure at the scribe suggests that the high degree of cathodic protection that zinc-rich MC urethane systems provide is not as high as that of inorganic zinc silicate coatings. After two years of outdoor exposure, all MC urethane coating systems remained in excellent condition and did not exhibit any failure.
The performance of the MC urethane coating systems on steel is likely to be affected by different chemical compositions, including the aromatic/aliphatic content ratio of the MC urethane binders, binder/ pigment content ratio, and particle-size distribution of pigment. Insufficient binder content and overly large pigment particles would result in early coating failure. It is believed that zinc grade and particle distribution play a more important role in primer performance than zinc content. Material testing and product quality control practices are necessary prior to coating application.
Shuang-Ling Chong is a research chemist in the Special Projects and Engineering Division, Office of Engineering R&D, Federal Highway Administration. Her 29 years of research experience covers photolysis, ion-molecule reactions, fractionation and characterization of organic materials in fossil fuels and petroleum, and identification of toxic organics and metals in coal-combustion residues. Dr. Chong has been conducting staff research since 1989 in paint testing, coating failure analysis, and evaluation of low volatile-organic-compound coating systems for steel bridges.
Yuan Yao is a chemist at Soil and Land Use Technology Inc., an on-site contractor at the Federal Highway Administration. She earned an M.S. degree in chemistry and her past experience includes waste water analysis.
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