Thermal decomposition kinetic study on one-pack formulations of epoxy resin cured with novel phosphorus-containing flame retardant latent curing agents

Several phosphorous-containing flame retardant latent curing agents newly derived from 4,4-diamino diphenyl sulfone (DDS) and 4,4-diamino diphenyl methane (DDM) were designed to cure epoxy resin alongside dicyandiamide (Dicy). The thermal decomposition and kinetics of cured one-packed systems were investigated in detail. The epoxy systems were studied under a nitrogen atmosphere by using thermal gravimetric analysis at different heating rates. The kinetics of thermal decomposition was evaluated by Flynn-Wall-Ozawa, Kissinger-Akahira-Sunose, and Starink methods. The correlation coefficient for all systems obtained by the three above methods was almost equal to one, indicating that these three methods were correctly chosen. The results demonstrated that the activation energy of epoxy/Dicy blended with different phosphorous-containing DDS- and DDM-derived curing agents at a lower degree of conversion decreased by incorporation of the agents while increasing at a higher degree of conversion. The thermodynamic parameters (Delta H*, Delta G*, and Delta S*) of the studied systems were calculated by the obtained activation energy via three kinetics methods. It was found that as the conversion degree and the enthalpy increased, the Gibbs free energy decreased, while entropy changed from negative to positive values.