The Study on Failure Process of Acrylic Polyurethane Coating under Alternate Wetting and Drying Condition

摘要

The failure process of an acrylic polyurethane coating in simulated sea water under a wet-dry cyclic condition and immersion condition was studied with the methods of electrochemical impedance spectroscopy (EIS), Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscope (SEM). The results show that the failure rate of acrylic polyurethane coating under the condition of wet-dry alternation is obviously greater than that under complete immersion. Under wet-dry condition both the porosity and the water absorptivity of the coating are also greater than those under complete immersion condition. Owing to the physical effect of wet-dry alternation, the fillers in the coating surface layer may fall off and result in micro-pores, which could multiply the defects in the coating and accelerate the coating degradation. FT-IR analysis shows that the isocyanate group in the coating not only participates in the curing of coating, but also hydrolyzes with water molecule. The C-O bonds fracture partly due to hydrolysis of the main molecule chains, which is one reason of the coating failure. A failure mode for acrylic polyurethane coating under a wet-dry cyclic condition is suggested.