Laboratory Evaluation of Waterborne Coatings on Steel
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2003-03-01
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Abstract:Ten commercial waterborne coatings formulated for protecting new steel (SSPC-SP 10) from corrosion were evaluated; they included five acrylic coatings, four epoxy coatings, and one polyurethane coating. The volatile organic compound content of all these waterborne coating materials was below 300 grams per liter. Several key chemical and physical properties were determined to investigate their effect on coating performance. In general, the binder-to-pigment weight ratio stayed above 1.0 for acrylic primers but remained below 1.0 for epoxy primers. A rapid infrared spectroscopic technique was developed in this study to distinguish styrene-modified acrylics from unmodified acrylics (vinyl acrylics). It was found that unmodified acrylics performed better than styrene-modified acrylics in terms of corrosion resistance at scribe. Most of the acrylic primers contained zinc phosphate that was otherwise not present in the epoxy primers. Virtually all the waterborne coatings showed adhesion strengths greater than 14 mega pascals. Both laboratory and outdoor tests were conducted for the performance evaluation. Two 3,000-h cyclic laboratory tests consisted of freeze, UV/condensation, salt-fog/dry-air cycles but 2 different salt solutions were used to generate salt fog-0.35 percent ammonium sulfate plus 0.05 percent sodium chloride (test A) and 5 percent sodium chloride (test B). Another set of test panels was exposed at Sea Isle City, NJ for 2 years for comparison purposes. Topcoat gloss and topcoat color all changed significantly after the tests. In general, little coating surface failures were observed, but all the coating systems developed creepage at an intentional scribe. The scribe creepage was used as the main parameter to compare coating performance; it was found to grow linearly with test time after the first detection time in all cases. The test results showed that the waterborne coating systems performed poorer than the zinc-rich systems. For the waterborne coatings, the performance ranked in the order of acrylics > epoxy > polyurethane. The performance of the currently used waterborne acrylic formulations has been significantly improved and the results are competitive with the best performing solvent-borne acrylic and epoxy barrier coatings. Furthermore, it is encouraging that properly formulated two-coat acrylic and epoxy systems can perform as well as generically-similar three-coat coating systems with different formulations. The analysis of the test results showed strong discrimination against zinc-rich systems by test A but not by test B. The cyclic freeze/UV-condensation/salt fog-dry air test that used 5 percent sodium chloride solution to generate salt fog was found to produce coating performance similar to that in salt-rich environment including zinc-rich and waterborne coatings.
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