Influence of substituents on the kinetics of epoxy/aromatic diamine resin systems

Eleven different epoxy/diamine systems, including tetraglycidyl-4,4′- diaminodiphenylmethane (TGDDM), triglycidyl p-aminophenol (TGAP), and diglycidyl ether of bisphenol A (DGEBA) with 4,4′-diaminodiphenylsulfone (DBS), diethyltoluenediamine (DETDA), dimethylthiotoluenediamine (DMTDA), and meta-phenylenediamine (m-PDA), were studied with near-infrared spectroscopy at different temperatures. The reactivities of the epoxies were determined and found to be in the following order when reacted with the same amine: DGEBA > TGAP > TGDDM. When the primary amine was reacted with the same epoxy, the order was DETDA > DDS > DMTDA; for the secondary amine, the order was DETDA > DMTDA > DDS. The relative reaction rates of the secondary amine to the primary amine were compared and discussed in terms of the structural differences and the corresponding substitution effect. It was concluded that the increase in the secondary amine reactivity of DETDA and DMTDA was caused by the deconjugation of the benzene-ring π electrons from the lone pair on the N atom. The overall order of the secondary amine relative reactivity was DMTDA > ; DETDA > DDS for the same epoxy and TGDDM > TGAP > DGEBA for the same amine. The m-PDA systems had no significant positive or negative substitution effects. Molecular orbital calculations were performed, and the results showed the most significant deconjugation effect in the secondary amine of DETDA.