Study on curing mechanism of DGEBA/nickel–imidazole system

The role of Im6NiBr2 on the polymerization of diglycidyl ether of bisphenol A (DGEBA) was examined. Scanning and isothermal calorimetry experiments were used to investigate the curing behavior over the range of curing catalyst concentration between 0.5 and 42 phr (parts per hundred). Based on the maximum values of reaction enthalpy and glass transition temperature (Tg), the best concentration of the curing catalyst was found to be 15 phr. One exothermic peak was observed at low and at high curing catalyst concentrations, while two exothermic peaks were observed at intermediate concentrations during the scanning calorimetry traces. It is suggested that there are two steps in the polymerization mechanism. The first step is the formation of 1:1 adduct between nickel–imidazole and epoxy oligomers. The second step is a chain growth polymerization reaction. It has been found that at high curing catalyst concentration the epoxy oligomers could react with nickel–imidazole and free imidazole ligand. In situ spectroscopy investigations showed that a fraction of nickel–imidazole was dissociated and then regenerated during the curing reaction. The regeneration can be explained by at least two disciplines of N-dealkylation and/or Hofmann elimination demonstrating the catalytic nature of the used nickel catalyst at certain conditions.