A modified kinetic model of the epoxy-amine reaction

Full Paper: The epoxy-amine reaction kinetics still remain an attractive field of investigation since these reactions form different thermosetting polymers being of a great practical importance. One of the most debatable problems of the epoxy-amine addition is the reactivity ratio of the primary to secondary amine hydrogens, also known as kinetic substitution effect (KSE). The analysis of the literature shows quite controversial data regarding the reactivity ratio, but less reactive secondary amine (or negative KSE) seems to be a more acceptable phenomenon. It can be ascribed to the steric hindrances neighboring the secondary amines formed during the reaction of the primary ones. On the other side, the kinetics of a given epoxy-amine reaction is often found either to corroborate an overall three molecular autocatalytic equation, representing a possible mechanistic scheme, or to obey a lower order kinetic model. From a mechanistic point of view, the last result indicates inconstant KSE and a lower over-all order corresponds to positive KSE. It is noteworthy that the reaction of diglycidyl ether of bisphenol a (DGEBA) with m-phenylenediamine (mPDA), investigated by us, behaves similarly even when the same experimental technique had been applied. In order to explain this inconstancy we have attempted to derive a modified kinetic model. It is based on the existing ones but it contains a semi-empirical term accounting for the solubility of the reaction components. We have shown that partial compatibility in the epoxy-amine system might significantly affect the course of the reaction. Moreover, it alternates the highly positive KSE of the reaction of DGEBA with mPDA, established by us.