Thermal-oxidative degradation of epoxy and epoxy-bismaleimide networks: kinetics and mechanism

The thermal oxidative stability of an epoxide/diamine network (TGDDM/DDS) and of an epoxide/diamine/bismaleimide IPN system (TGDDM/DDS/BMI) have been investigated by thermogravimetric analysis (TGA), dynamic-mechanical thermal analysis (DMTA) and isothermal, time-resolved FTIR spectroscopy. Both TGA and DMTA data revealed a considerable enhancement of the stability of the material as the bismaleimide content in the system is increased. The FTIR measurements allowed us to monitor in-situ the degradation process and to obtain reliable kinetic data. These, together with the analysis of the time evolution of the infrared spectra, permitted to propose likely mechanisms to account for the experimental observations. It was found that in the IPN system the epoxide network degrades with the same mechanism occurring in the pure epoxy resin, albeit at an increased rate. No evidence has been found of alternative pathways introduced in the system by the presence of the bismaleimide component. The bismaleimide network was also found to undergo significant degradation in the IPN system, but to a considerably lesser extent than the more oxygen sensitive groups of the TGDDM/DDS network. The higher stability of the bismaleimide component represents the underlying reason for the enhanced thermal-oxidative resistance of the ternary system with respect to the binary TGDDM/DDS resin.