Agro-byproduct-based hydrophobic polyurethane films/coatings: synthesis, characterization and electrochemical impedance spectroscopic analysis

Bio-based polyurethanes (PU) prepared through green routes using technical cashew nut shell liquid (TechCNSL) for anti-corrosive application could protect infrastructures and addresses environmental concerns. The synthesis was carried out in situ by an acid-catalyzed condensation reaction of TechCNSL and formaldehyde (F) at 120 °C. The final structure was obtained through addition polymerization of TechCNSL-F with toluene diisocyanate (TDI) at room temperature (OH:NCO ratios: 1:0.8, 1:1 and 1:1.2) to form TechCNSL-F-PU. The characteristic properties of the coatings were influenced as a function of free –OH and –NCO groups that is influenced by the composition of TechCNSL used. The high coating resistance (R_c) and change in solution resistance (R_s) values along with lower coating capacitance (C_c) values supporting sufficient protection ability and compactness to the metallic substrate was determined by non-destructive electrochemical impedance spectroscopy (EIS) studies. Structural and progress of the reaction was evaluated by ATR/FTIR, whereas XRD revealed the amorphous nature of the hydrophobic films/coatings with a contact angle of 100° for TechCNSL-F-PU1:1. The variation in thermal stability with TDI concentration and glass transition (T_g) temperature was based on the degree of cross-linking density of the systems. The degree of cross-linking was calculated through estimation of gel content, which varied from 96 to 98% based on the concentration of TDI with thermal stability by max 265 °C. TechCNSL-F-PU1:1 shows excellent films/coatings adhesion, anticorrosion performance, chemical and mechanical resistance in the various chemical environment (water, 3.5% of HCl, NaCl and NaOH) and showed the best performance. These thermally stable and mechanically robust PU films/coatings can be projected for the corrosion protective applications. Graphical abstract: [Figure not available: see fulltext.].