Effect of structure of aromatic diamines on curing characteristics, thermal stability, and mechanical properties of epoxy resins. I

The curing behavior of diglycidyl ether of bisphenol-A (DGEBA) with aromatic diamines having aryl-ether, aryl-ether-carbonyl, and aryl-ether-sulfone linkages was studied using differential scanning calorimetry (DSC). Aromatic diamines such as 1,3-bis(aminophenoxy)benzene (R), 1,4-bis( aminophenoxy)benzene (H), 2,2'-bis[4-(4-aminophenoxy)phenyl]propane (B), 4,4'-bis(4-aminophenoxy)benzophenone (P), and his [4-(4-aminophenoxy) phenyl] sulfone (S) were synthesized and characterized in the laboratory. Curing of DGEBA was done using both stoichiometric and nonstoichiometric amounts of diamines and the reaction was monitored using DSC. The reactivity of the diamines depended on the structure. The presence of electron withdrawing groups, even though significantly apart from the reaction site, reduced the nucleophilicity. No significant change was observed in the activation energy for curing, which was around 56 +/- 2 kJ/mol. The glass transition temperature of the epoxy network depended on the structure and was higher when diamines P and S were used in comparison to diamines R, H, and B. The cured resins were stable up to 300 degrees C, and maximum char yield (i.e., 32% at 600 degrees C) was obtained in DGEBA cured with diamine P. The room temperature mechanical properties only changed marginally with the structure of the diamines. (C) 1998 John Wiley & Sons, Inc.