Synthesis and characterization of thermally curable guaiacol based poly(benzoxazine-urethane) coating for corrosion protection on mild steel

Biobased hydroxyl functional benzoxazine monomer was synthesized from Guaiacol (lignin derivative) by Mannich condensation reaction using solvent less method. The structure of hydroxyl functional benzoxazine monomer was elucidated from FTIR, H-1 NMR, C-13 NMR spectroscopy and mass spectrometry. The ring opening polymerization conversion with respect to time was estimated by H-1 NMR. Thermal curing behaviour of benzoxazine monomer was studied by DSC. The hydroxyl functional benzoxazine monomer was cured with isocyanate hardener in varying concentrations based on equivalent weight to prepared the poly(benzoxazine-urethane) (BZPU) coatings. After thermal curing, BZ and BZPU coatings were examined by FTIR analysis. The effect of varying concentrations of isocyanate hardener with benzoxazine monomer on mechanical, chemical, solvent, thermal and anticorrosive properties of coatings was investigated. The study revealed that, an increased concentration of urethane moieties in BZPU coatings improved the mechanical, chemical and thermal properties, compared to pure polybenzoxazine (BZ) coating. The effect of urethane content on corrosion resistance of BZPU coatings were investigated by salt spray and electrochemical impedance spectroscopy (EIS) technique. The results shown that corrosion resistance of BZPU coating enhanced with increasing urethane content in polymer backbone. The protection efficiency values for BZPU coatings were found more than 97%. (C) 2016 Elsevier Ltd. All rights reserved.