Crambe oil, castor oil and two different proportions of crosslinking agent (Trimethylolpropane) were made to react to form two types of hydroxylated polyesters. Hexamethylene diisocyanate was reacted with castor oil and oxygenated organic compounds to produce a prepolymer. The polyesters were combined with prepolymer in two different proportions to result four polyurethane resins which were applied on AA1200 aluminum alloy surface, giving to the aluminum alloy high corrosion resistance in 3.5% NaCl aqueous solution at 25 ^oC [1]. This study deals with the wetting characteristic of bare and coated aluminum surfaces by measuring the dynamic contact angle. The determination of the contact angle has helped to interpret the hydrophobic and hydrophilic character of the surface. Surface free energy components (γ_s, γ_s^p, γ_s^d), and adhesion work (W_A) values were calculated by the OWRK method (Owens-Wendt-Rabel and Kaelble) according to the contact angle (Ө) data. For comparison, three liquids with different polarities (distilled water, ethylene glycol and ethanol) were used on the surface of the aluminum alloy with and without organic coating. The results of the contact angle measurements allowed to evaluate the wettability of the substrate surface (with and without coating) and to estimate the free surface energy. The greater the contact angle of the liquid in the organic coating, the more hydrophobic the surface is, and consequently less susceptible to wetting and corrosion. Compared to the substrate (AA1200 aluminum) contact angle of 102.4°, the coatings are less hydrophobic (PEC2 H3 1:2 contact angle of 73.6° and PEC2 H3 1:3 equal to 82.0°). Already the coatings have higher surface energy: substrate (22.2 mN/m), PEC2 H3 1:2 (31.3 mN/m) and PEC2 H3 1:3 (25.4 mN/m). Changes in contact angle and surface free energy are associated with the possibility of forming more hydrogen bonds with the coating.

Reference

[1] J.V. Nardeli, D.V. Snihirova, C.S. Fugivara, M.F. Montemor, E.R.P. Pinto, Y. Messaddecq, A.V. Benedetti, Localised corrosion assessement of crambe-oil-based polyurethane coatings applied on the ASTM 1200 aluminum alloy, Corros. Sci. 111 (2016) 422-435.

Acknowledgement

Process FAPESP: 2015/10554-9.