Thermal properties and fracture toughness of epoxy resins cured by phosphonium and pyrazinium salts as latent cationic initiators

In this work, the latent thermal cationic initiators triphenyl benzyl phosphonium hexafluoroantimonate (TBPH) and benzyl-2-methylpyrazinium hexafluoroantimonate (BMPH) were newly synthesized and characterized with IR, ¹H NMR, and P NMR spectroscopy. The thermal and mechanical properties of difunctional epoxy [diglycidyl ether of bisphenol A (DGEBA)] resins cured by 1 phr of either TBPH or BMPH were investigated. The DGEBA/TBPH system showed a higher curing temperature and a higher critical stress intensity factor than the epoxy/BMPH system. This could be interpreted in terms of the slow thermal diffusion rate and bulk structure of the four phenyl groups in TBPH. However, the decomposition activation energy derived from the Coats-Redfern method was lower for epoxy/TBPH. This result was probably due to the fact that a broken short-chain structure was developed by the steric hindrance of TBPH in the difunctional epoxy resin.