Epoxy-Aromatic Diamine Kinetics. Part 1. Modeling and Influence of the Diamine Structure

Abstract

The reactivities of different aromatic diamines, i.e. 4,4'-diaminodiphenyl sulfone (DDS), 4,4'-methylenedianiline (MDA), 4,4'-methylenebis[2,6-diethylaniline] (MDEA), and 4,4'-methylenebis[3-chloro-2,6-diethylaniline] (MCDEA), cured with an epoxy prepolymer, the diglycidyl ether of Bisphenol A (DGEBA), were compared and correlated to their chemical structures. The reactivities of the diamines are in the following order : MDA > MDEA > DDS > MCDEA. Although MDA, MDEA, and MCDEA exhibit very different reactivities, the ratio of the reactivity of secondary to primary amine hydrogens (k 2 /k 1 ) is found to be the same r = 0.65. On the other hand, the reactivity ratio of DDS is found to be r = 0.45. This lower value may be the result of the lower number of stable conformations afforded by the -SO 2 - group in comparison to the -CH 2 - group. No temperature dependence was observed. The DGEBA-DDS and DGEBA-MCDEA kinetics were fully characterized as regards conversions and T g measurements with time at different temperatures. Relationships between the T g and x were validated for both systems. Kinetics were modeled throughout the whole range of cure. For the DGEBA-MCDEA system, etherification reactions were found to occur and were then integrated into the kinetic model. The effect of diffusion control was incorporated by modifying the overall rate constant according to the Rabinovitch model. The Adam-Gibbs theory was preferred to express the temperature dependence of the diffusion rate constant. The kinetic models have been satisfactorily extended to the isothermal cure of an epoxy with a binary mixture of diamines.