Amidoamine: Synthesis, disparity in cure with epoxy resins between bulk and solvent systems, and structure–property relationships of its epoxy‐based coatings

Amidoamines are widely used as crosslinkers for epoxy resins in protective coatings on metal substrates; however, their cure chemistry is not well elaborated in the technical literature. During cure, the epoxy–amine and epoxy–amide NH reactions could be accompanied by epoxy–hydroxyl etherification, epoxy–epoxy homopolymerization, and reaction between hydroxyls and amide moiety to form in situ ester and amine. To understand the epoxy–amidoamine cure chemistry and correlate it with coating performance properties, amidoamines of known structure are required. Therefore, amidoamines were synthesized by reacting dimer fatty acids with diethylene triamine. When these amidoamines were cured with epoxy resins, discrepancies were observed between systems cured at ambient and thermal conditions. The presence of solvents were seen to greatly affect the rate of epoxy–amidoamine cure at ambient, and before and after vitrification. Near‐IR and mid‐IR spectroscopy studies indicated that side reactions occurred to some extent during cure. No reaction was noted between the amide NH moiety and the epoxide group below 150°C whereas ester formation was noted above 120°C. Solvent‐based clear epoxy–amidoamine coatings formulated at an epoxy:amine equivalent ratio 1.15 passed basic organic coatings evaluation tests. POLYM. ENG. SCI., 59:E69–E81, 2019. © 2018 Society of Plastics Engineers