Influence of the accelerator concentration on the curing reaction of an epoxy-anhydride system

Abstract

The effect of accelerator content on the curing reaction of an epoxy resin based on diglycidyl ether of bisphenol A (DGEBA) with methyl-tetrahydrophthalic anhydride was studied by DSC. The uncatalysed curing occurs at high temperature (between 190 and 310°C) with thermal degradation. The addition of accelerator which is a tertiary amine catalyst agent, namely dimethylbenzylamine (DMBA), causes two exothermic peaks. The cure extent and the position of the peaks depend on the accelerator content. The first peak, which is sharp and well defined, appears between 80 and 200°C and may be attributed to the catalysed curing. The second peak, which is broad, only appears for low accelerator content (lower than 1 pbw) in the zone of uncatalysed curing (between 200 and 320°C), and may be attributed to the uncatalysed curing. The activation energy corresponding to the first exothermic peak in the catalysed curing, calculated by the Kissinger method, decreases with the accelerator content. Kinetic analysis performed by Málek's method shows that the autocatalytic model (two-parameter S̆esták-Berggren equation) can describe satisfactorily the kinetics of the catalysed and uncatalysed curing. In the catalysed system, the parameters m and n increase slightly with the accelerator content, and the pre-exponential factor, ln A, undergoes a slight decrease. The maximum T g of the fully cured epoxy obtained by isothermal curing at 110°C in catalysed systems shows no significant changes. These results mean that the differences observed in the kinetics of curing between catalysed systems do not imply significant changes in the structure of the network of the epoxy resin.