Thermal curing of epoxy resins at lower temperature using 4-(methylamino)pyridine derivatives as novel thermal latent curing agents

To expand the application fields of epoxy resins, there has been a growing demand for thermal latent curing agents that combine a lower curing temperature with a long storage lifetime for a one-component epoxy formulation. Herein, we present the use of novel thermal latent curing agents for epoxy resins that are effective at lower curing temperatures compared to epoxy resins containing imidazoles as curing agents—they are based on 4-(methylamino)pyridine (4MAPy), with reactivity suppressed by different amide protecting groups. We revealed reaction mechanism of epoxy polymerization by the thermal latent 4MAPy, using Matrix-assisted laser desorption ionization–time-of-flight mass spectrometry, Fourier transform infrared spectroscopy (FT-IR), and ¹H NMR analyses of the model polymerization of phenyl glycidyl ether. The amide protecting groups decomposed to form the highly reactive 4MAPy by nucleophilic acyl substitution with the propagating alkoxide. Furthermore, FT-IR data and pencil hardness tests revealed that thermal latent 4MAPy derivatives, particularly those with electron-rich pyridine rings, can cure epoxy resins at lower curing temperatures, notably 90°C. The thermal latent 4MAPy derivatives with epoxy resin exhibited a storage lifetime of at least 6 days at 3°C, in contrast to 4MAPy with epoxy resin that cured within a day after mixing.