Laboratory evaluation of waterborne coatings on steel Publication No. FHWA-RD-03-032 - Communication Product Updates - Brief Article

Public Roads, Sept-Oct, 2003 by Zac Ellis

Researchers evaluated 10 commercial waterborne coatings formulated to protect new steel (SSPC-SP 10) from corrosion. They included five acrylic coatings, four epoxy coatings, and one polyurethane coating. The volatile organic compound content of all the coating materials was below 300 grams per liter (2.5 pounds per gallon). Researchers determined several key chemical and physical properties to investigate their effects 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. The researchers developed a rapid infrared spectroscopic technique to distinguish styrene-modified acrylics from unmodified (vinyl) acrylics. The performance evaluation involved both laboratory and outdoor tests.

The performance of the waterborne acrylic formulations currently in use has been improved significantly, and the results are competitive with the best performing, solvent-borne acrylic and epoxy barrier coatings. Properly formulated two-coat acrylic and epoxy systems can perform as well as generally similar three-coat 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 produced a coating performance similar to that in a salt-rich environment including zinc-rich and waterborne coatings.