Hybrid waterborne polyurethane coating behavior in saline solution

Waterborne polyurethane/acrylate (WPUA) dispersion was synthesized via a prepolymer self-emulsifying method, using bisphenol-A-glycidyl dimethacrylate (Bis-GMA) as a chain extender and grafting agent for styrene and butyl acrylate monomers. The degradation of WPUA coatings on mild steel in a 3.5 wt% NaCl solution was investigated through electrochemical impedance spectroscopy and open circuit potential measurements. The water uptake estimated from the Brasher-Kingsbury relation, the intersection of Bode plots (IBP number), and electrical equivalent circuits (ECC) has been analyzed during the degradation process. Particle size (DLS) and morphological (TEM) analyses revealed that the WPUA dispersion exhibited nanometric particle sizes with a core-shell structure. The thermal (TGA/DTG) properties and cross-cut adhesion of the WPUA coatings were greatly enhanced compared to the blank WPU. Contrary to typical expectations, the impedance modulus in the low-frequency range (lZ 0.01Hz l) did not decrease with immersion time for all samples; this effect is attributed to the polymer network characteristics. The degradation mechanisms of the WPUA coatings involve three steps: an initial coating failure as a consequence of the entry of water; in the second step, water absorption continues until a critical level is reached. During this stage, no significant changes in coating resistance were detected. Finally, sudden changes in the monitored parameters indicate the loss of the protective properties. • Waterborne polyurethane/acrylate (WPUA) dispersion was synthesized using Bis-GMA. • Bis-GMA improves thermal stability and cross-cut adhesion of WPUA coatings. • The degradation of WPUA coatings in saline solution was studied by EIS. • The interpenetrating polymeric network influenced the coating degradation mechanism. • The degradation of WPUA coatings in saline solution occurs in three distinct steps.