Poly(ethylene oxide)-modified waterborne epoxy coatings for turbulent-flow drag reduction: Effects of formulation on surface properties and performance

Polymer drag reduction is well established in internal flows, but its application to external water flows remains challenging because the near-wall fluid is continuously renewed, making it difficult to retain active polymer near the solid surface. Inspired by the hydrated polymer-rich surface of fish mucus, poly(ethylene oxide)-modified waterborne epoxy (WEP-PEO) coatings were developed as functional coatings for drag reduction in turbulent water flow. The effects of curing-agent ratio, epoxy resin emulsion concentration, and PEO loading on coating chemistry, viscoelastic response, surface morphology, roughness, wettability, and drag-reduction performance were systematically evaluated. FTIR analysis indicated the presence of PEO-related ether groups and hydroxyl-containing structures, while oscillatory rheology showed an elastic-dominated response strongly affected by the curing-agent ratio. Post-flushing ATR-FTIR showed that PEO-associated structures remained detectable near the optimized coating surface after 20 min of turbulent flushing, whereas downstream TOC analysis confirmed the simultaneous release of polymer-associated organic species into the water phase. The formulation containing a curing-agent-to-epoxy-emulsion ratio of 2:16, 40 wt% epoxy resin emulsion, and 1.75 wt% PEO exhibited a continuous surface, relatively low roughness (Ra=1.88μm[jls-end-space/]), favorable wettability, and a balanced viscoelastic response. At Re = 22,925, the optimized coating achieved drag-reduction rates of approximately 17.5% in the coated section and 22.0% downstream. At Re = 11,462, the initial drag reduction reached approximately 21% and remained close to 19% after 20 min of flushing, while performance decreased at higher Re. These results clarify the relationship among coating formulation, film formation, surface properties, and drag-reduction performance, providing a basis for designing waterborne epoxy functional coatings for turbulent water flow.