Amidoamine: Synthesis, Disparity in Cure with Epoxy Resins Between Bulk and Solvent Systems, and Structure–Property Relationships of Its Epoxy-Based Coatings Monoj Pramanik , Mark Early, Steven Wand, Diana Gottschalk, Sharathkumar K. Mendon, James W. Rawlins School of Polymers and High Performance Materials, The University of Southern Mississippi, Hattiesburg, Mississippi 39406 Amidoamines are widely used as crosslinkers for epoxy resins in protective coatings on metal substrates; how- ever, 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 homopolyme- rization, and reaction between hydroxyls and amide moi- ety 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 syn- thesized by reacting dimer fatty acids with diethylene tria- mine. 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 vitrifi- cation. Near-IR and mid-IR spectroscopy studies indi- cated that side reactions occurred to some extent during cure. No reaction was noted between the amide NH moi- ety and the epoxide group below 1508C whereas ester for- mation was noted above 1208C. Solvent-based clear epoxy–amidoamine coatings formulated at an epoxy:- amine equivalent ratio 1.15 passed basic organic coatings evaluation tests. POLYM. ENG. SCI., 00:000–000, 2018. V C 2018 Society of Plastics Engineers INTRODUCTION Epoxy prepolymers are widely used in coatings and adhe- sives because of their ease of processing, low shrinkage upon curing, good thermal and mechanical stability, high resistance to solvents, and promising anticorrosive properties [1–6]. Amine curatives and reactive amidoamines are popular building blocks for matrices in two component epoxy coatings [3, 7–15]. Such amidoamines are synthesized by reacting fatty acids with amines [16–19]. Coatings based on epoxy resins cured with dimer fatty acid-based amidoamines offer the advantages of low cost, excel- lent flexibility, and high hydrophobicity. Since the properties of epoxy–amidoamine coatings depend on the extent of cure, it was necessary to study its cure kinetics to establish structure– property relationships. It is well established that amine moieties (-NH and -NH 2 ) in amidoamines react with epoxy moieties at ambient and elevated temperatures. However, the reaction between NH of - (C 5 O)NH moieties and epoxy groups at ambient or elevated temperatures has hardly been studied. Cure kinetic study of epoxy–amidoamine systems via differential scanning calorimetry (DSC) only dispenses the overall extent of reaction without dif- ferentiating between the individual component reactions [18, 20–27]. The evaluation of epoxy–amidoamine reactions via mid- IR (600–4,000 cm 21 ) spectroscopy [20, 28] has been compro- mised by the fact that epoxy resins and amidoamines exhibit overlapping bands in the mid-IR region. Prime and Sacher [20] reported that the cure of epoxy–amidoamine blends were charac- terized by the reaction between amine and epoxy moieties at ambient temperature, and the formation of ester and amine moi- eties via amide-alcohol reactions at elevated temperatures. Zhong and Guo [28] studied cure kinetics of epoxy and nylon blends via DSC and mid-IR spectroscopy, and reported that the reaction between epoxy groups and the amide moiety of nylon, and ester formation via amide-hydroxyl reaction occurred above 2008C. The extent of side reactions is important as it alters the curing agent functionality and induces variations in crosslink density as a function of the thermal cure profile. When amidoamines react with epoxy resins in bulk at ambi- ent conditions, progressive cure results in system solidification, ultimately resulting in vitrification, that is, the progressive loss of functional groups slows down the molecular motion and the rate of crosslinking is typically reduced by three to four orders of magnitude (cure is almost halted). In solvent-based epoxy– amidoamine blends, the solvents usually impart enough mobility for virtually all the groups to react before complete solvent evaporation (i.e., elongated open times without vitrification), and almost complete epoxy conversion is noted at ambient conditions. Epoxy–amine blend cure involves four reactions: epoxy–pri- mary amine, epoxy–secondary amine, epoxy–hydroxyl etherifi- cation, and epoxy–epoxy homopolymerization, that are governed by their energy barriers of 55–60, 71, 104, and 170 kJ/ mol, respectively (some variation in energy barriers is likely depending on the physical state and chemical structures of monomers) [29, 30]. Noting that near-IR (4,000–8,000 cm 21 ) spectra of epoxy–amine systems are characterized by distinct bands for epoxide, primary amine, phenyl, and combined pri- mary–secondary amine and hydroxyl groups, we monitored the cure kinetics of epoxy–amine blends and showed that epoxy– hydroxyl etherification and epoxy–epoxy homopolymerization Correspondence to: J. W. Rawlins; e-mail: james.rawlins@usm.edu Additional Supporting Information may be found in the online version of this article. Contract grant sponsor: The United States Air Force through funding by the Department of Defense (DoD); contract grant number: FA7000-13-2-0022 and FA7000-14-2-0011. DOI 10.1002/pen.24858 Published online in Wiley Online Library (wileyonlinelibrary.com). V C 2018 Society of Plastics Engineers POLYMER ENGINEERING AND SCIENCE—2018