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Water-borne polyurethane coatings approach solvent-borne durability on steel bridges
A long-term comparative study including three years of outdoor marine exposure finds that commercial water-borne polyurethane coating systems approach the anticorrosive performance of conventional solvent-borne systems on steel bridge substrates, though differences emerge under accelerated cyclic corrosion testing.
Most steel bridges worldwide are protected with solvent-borne heavy-duty coating systems. However, the health and environmental hazards associated with high volatile organic compound (VOC) emissions are driving growing interest in water-borne alternatives. A key barrier to wider adoption has been the limited availability of long-term performance data and field application experience for water-borne systems. A new study now provides a systematic durability comparison between a standard-specified commercial water-borne polyurethane system and a conventional solvent-borne counterpart.
Coated steel panels were subjected to two indoor accelerated tests – 16,000 hours of fluorescent ultraviolet light exposure and 13,200 hours of cyclic corrosion testing – as well as a three-year outdoor marine exposure trial. Performance was assessed on the basis of gloss retention, colour difference, chalking, rust area and adhesion strength.
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Accelerated tests reveal primer-dependent differences
Both coating systems showed convergent gloss retention behaviour, declining to less than 20 % after a certain exposure period. However, notable differences emerged in the cyclic corrosion test, where rust propagation on water-borne panels progressed approximately 75 % faster than on solvent-borne panels. The researchers attributed this divergence to the different primer mechanisms employed in the two systems rather than to inherent weaknesses of the water-borne topcoat.
Importantly, after three years of real-world outdoor marine weathering, the difference in rust propagation rates between the two systems was minimal. Adhesion strength satisfied the minimum criteria required for both coating systems throughout the test programme.
Outdoor data support water-borne adoption for bridge protection
The findings suggest that accelerated indoor corrosion tests may overstate the performance gap between water-borne and solvent-borne systems compared to actual field exposure conditions. For bridge operators and coating specifiers considering the transition to lower-VOC systems, the study provides valuable evidence that commercial water-borne polyurethane coatings can deliver comparable long-term durability in marine environments – provided that primer selection and system design are carefully matched to the application requirements.
Source: Lee, S.-H. et al., Long-term durability comparison of commercial waterborne and solvent-based polyurethane coatings for steel bridges. Progress in Organic Coatings 109924 (2025).
