Thermal curing of para-phenylenediamine benzoxazine for barrier coating applications on 1050 aluminum alloys

Polybenzoxazine coatings have been applied by spin coating of a solution, prepared from a laboratory synthesized phenol, para-phenylenediamine benzoxazine (P-pPDA), on 1050 aluminum alloy. A heat treatment was performed on the coating to polymerize the benzoxazine monomer by a ring opening mechanism attested for by Fourier Transform Infrared spectroscopy (FT-IR) and followed by Differential Scanning Calorimetry (DSC). Dielectric Analysis (DEA) showed curing to proceed at a temperature range up to 230 °C, without thermal decomposition taking place, as has already been reported in the literature for commercial bisphenol-a benzoxazines (Ba-A). The barrier protection has been evaluated by Electrochemical Impedance Spectroscopy (EIS) for 30 days in a sodium chloride solution (0.1 M). The results were fitted with equivalent electrical circuits in order to understand the electrochemical behavior of such a system. The initial barrier properties of both precursor and polymerized coatings appear to be excellent for coatings having a thickness of only 5 μm. Though delamination occurs in the polymerized coating during saline tests after several days of immersion, such kind of coatings have remarkable properties with respect to epoxy coatings as they show a high geometrical stability during curing and a very low water uptake during saline tests.