Sustainable cardanol-based multifunctional carboxyl curing agents for epoxy coatings: Si–S synergism

High-performance epoxy coatings were prepared by crosslinking commercial diglycidyl ether of bisphenol-A with silicon–sulfur containing di- and tetra-functional carboxyl curing agents based on cardanol. The curing agents were synthesized and products were analyzed by chemical, spectroscopic, and chromatographic techniques. In the next part, coatings were formulated by varying the ratio of epoxy resin to curing agents on equivalent basis, such as 1:0.6, 1:0.8, and 1:1. The coatings were applied on mild steel panels and cured at 150°C for 30 min. The resultant coatings were evaluated for performance properties including mechanical, chemical, optical, thermal anticorrosive, and flame retardant properties. It was observed that with an increase in concentration of silicon–sulfur containing curing agents, water contact angles of the coatings substantially increased. Moreover, electrochemical impedance spectroscopy, Tafel analysis, and salt spray studies revealed that anticorrosive properties of the coatings improved with an increase in concentration of silicon and sulfur. In spite of synergistic effect of silicon–sulfur, only marginal improvement in flame retardant properties was observed.